Draft 

ADVISORY BOARD ON 

RADIATION AND WORKER HEALTH 

National Institute for Occupational Safety and Health 

A FOCUSED REVIEW OF OPERATIONS AND THORIUM EXPOSURES 

AT THE DOW CHEMICAL COMPANY MADISON PLANT

Contract No. 200-2004-03805 

Task Order No. 5 

SCA-SEC-TASK5-0057

Prepared by 

S. Cohen & Associates 

1608 Spring Hill Road, Suite 400 

Vienna, Virginia 22182 

August 2007 

Disclaimer 

This document is made available in accordance with the unanimous desire of the Advisory Board 

on Radiation and Worker Health (ABRWH) to maintain all possible openness in its 

deliberations. However, the ABRWH and its contractor, SC&A, caution the reader that at the 

time of its release, this report is pre-decisional and has not been reviewed by the Board for 

factual accuracy or applicability within the requirements of 42 CFR 82. This implies that once 

reviewed by the ABRWH, the Board’s position may differ from the report’s conclusions. Thus, 

the reader should be cautioned that this report is for information only and that premature 

interpretations regarding its conclusions are unwarranted.
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Document No. 

SCA-SEC-TASK5-0057 

Effective Date: 

Draft — August 24, 2007 

S. COHEN & ASSOCIATES:

Technical Support for the Advisory Board on 

Radiation & Worker Health Review of 

NIOSH Dose Reconstruction Program Revision No. 

0 – Draft 

A Focused Review of Operations and Thorium 

Exposures at the Dow Chemical Company 

Madison Plant

Page 2 of 66 

Task Manager: 

________________________ Date: ___________ 

Charles (Chick) Phillips 

Project Manager: 

________________________ Date: ___________ 

John Mauro, PhD, CHP 

Supersedes: 

N/A 
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TABLE OF CONTENTS 

1.0 Background..........................................................................................................................5 

2.0 Operations at Dow - Madison..............................................................................................7 

2.1 Radioactive Materials Handled................................................................................9 

2.2 Alloy Melting and Casting.....................................................................................10 

2.3 Alloy Fabrication ...................................................................................................21 

2.4 Sampling Techniques (circa 1956) ........................................................................26 

2.4.1 Thorium Metal ...........................................................................................26 

2.4.2 Alpha and Beta Activity.............................................................................26 

3.0 Internal Dosimetric Implications of Thorium Operations .................................................28 

3.1 Mg-Th Hardener Production..................................................................................28 

3.2 Melting of Alloys Using Mg-Th Hardener ............................................................30 

3.3 Mg-Th Alloy Fire...................................................................................................30 

3.4 Alloy Production....................................................................................................30 

3.5 Alloy Fabrication ...................................................................................................31 

4.0 Followup on Thorium Exposures After 1960 ....................................................................32 

4.1 June 20, 2007, Workers Meeting ...........................................................................32 

4.2 Thorium Alloy Production and Distribution to DOE/AWE Sites .........................34 

4.3 Thorium or Other Radioactive Materials Processed at Dow Madison ..................35 

4.4 Thorium Exposure Conditions beyond 1960 .........................................................36 

5.0 References..........................................................................................................................38 

Attachment A: Magnesium-Thorium Alloy Designations............................................................41 

Attachment B: Excerpt from FUSRAP Close-out Report for Dow Madison Site, 

September 2001 .................................................................................................................43 

Attachment C: Sections 14, 15, and 16 Excerpted from USAEC Inspection Report, 

August 4, 1660...................................................................................................................45 

Attachment D: Mg-Th System......................................................................................................49 

Attachment E: Th-232 Decay Chain.............................................................................................51 

Attachment F: Summary of Monitoring Results at Dow Madison...............................................52 

Attachment G: Use of Mg-Th Alloys at Rocky Flats ...................................................................65 
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LIST OF TABLES 

Table 1. Thorium Inventory at Dow Madison .................................................................... 10 

Table 2. Air-Sampling Measurements during Mg-Th Hardener Production...................... 12 

Table 3. Levels of Radioactivity Associated with Melting Alloys Using Mg-Th 

Hardener................................................................................................................ 13 

Table 4. Results of Air Sampling Measurements Made During a Mg-Th Alloy Fire ........ 15 

Table 5. Air Sampling Measurements Taken During Melting and Casting........................ 18 

Table 6. Melting Log for Alloy HK31................................................................................ 20 

Table 7. Air Sampling During the Chemical Milling of HK31 .......................................... 24 

LIST OF FIGURES 

Figure 1. The Dow Madison Plant ......................................................................................... 8 

Figure 2. Pot Room Schematic............................................................................................. 17 

Figure 3. Results of Air Samples Taken During Processing Thorium-Magnesium 

Alloys.................................................................................................................... 22 

Figure 4. Variation of Activity in Natural Thorium after a Single Purification................... 23 
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1.0 BACKGROUND 

On November 27, 2006, the law firm of Simmons-Cooper, LLC, filed a Special Exposure Cohort 

(SEC) Petition with the National Institute for Occupational Safety and Health (NIOSH) Office of 

Compensation and Support (OCAS) in behalf of workers at the Dow Chemical Company (DOW) 

plant in Madison, Illinois. On December 14, 2006, the Department of Health and Human 

Services published a notice in the Federal Register (71 FR 75258) indicating that the federal 

government had made a decision to evaluate the Dow employees petition to determine if the 

group should be included in an SEC. The period of employment for the proposed SEC was 

defined as January 1, 1957, through December 31, 1960. The period was chosen because during 

that timeframe, Dow did some uranium fabrication work related to Atomic Energy Commission 

(AEC) programs.1

 For the defined period, it is possible that some Dow workers received 

exposure to uranium (and daughter products) from the AEC uranium fabrication work, and to 

thorium from commercial activities related to the routine commercial melting, casting, and 

fabrication of Mg-Th alloys. 

 

As required by both Energy Employees Occupational Illness Compensation Program Act of 2000 

(EEOICPA) and Title 42, Part 83 of the Code of Federal Regulations (42 CFR Part 83), NIOSH 

issued SEC Petition Evaluation Report for SEC-00079 (NIOSH 2007) on April 13, 2007. In its 

evaluation, NIOSH decided that both uranium and thorium doses need to be considered during 

the time period of January 1, 1957, through December 31, 1960. The evaluation concluded the 

following: 

NIOSH has documented herein that it cannot complete the dose reconstructions 

related to this petition where doses resulted from exposure to thoriumcontaining materials. The basis of this finding is specified in this report, which 

demonstrates that NIOSH does not have access to sufficient information to 

estimate with sufficient accuracy either the maximum radiation dose incurred by 

any member of the class or to estimate such radiation doses more precisely than a 

maximum dose estimate. Members of this class at the Dow Chemical Company 

site in Madison, Illinois, may have received unmonitored internal and external 

radiological exposures from thorium radionuclides at the plant. NIOSH lacks 

sufficient information, which includes sufficient personnel and workplace 

monitoring data and radiological source term information, to allow NIOSH to 

estimate the potential total internal thorium exposures to which the proposed class 

may have been exposed. 

With the data currently available to NIOSH, it is feasible to reconstruct with 

sufficient accuracy the external and internal doses resulting from exposure to 

uranium metal during the Dow Madison AWE operational period (January 1, 1957 

1

 On March 15, 1957, Subcontract No. 25034 was implemented between the Uranium Division of Mallinckrodt 

Chemical Works and the Madison Division of Dow Chemical Company to do research and development work on 

gamma phase extrusion of uranium metal. No contractual evidence of prior work has been developed. However, a 

Record of Contact prepared by Spectrulite Consortium, Inc., quotes a former Dow employee as stating, “The work 

was done in the plant extrusion department beginning in 1955 for a period of about two years—may have been 

longer” (Young 1988). 
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through December 31, 1960), and during the residual radiation period (January 1, 

1961 through December 31, 1998). NIOSH also considers the reconstruction of 

medical dose for Dow Madison workers to be feasible. 

At a meeting of the Advisory Board on Radiation and Worker Health (Advisory Board) in 

Mason, Ohio, on February 9–11, 2007, SC&A, under its continuing support to the Advisory 

Board, was tasked to review the SEC petition and the NIOSH evaluation of the petition. 

Subsequently, at the Advisory Board Meeting in Denver, Colorado, on May 2, 2007, SC&A 

was asked to evaluate thorium exposures from 1961 forward at the Dow (Madison) plant and to 

review newly available material on the Madison site. This material consisted of approximately 

700 pages of information obtained by NIOSH from Dow Chemical Company and made 

available on the “O” drive on May 1, 2007. This report to the Advisory Board describes 

relevant activities at the Dow Plant for the defined time period and SC&A’s attempt to further 

clarify thorium exposures from 1961 forward. 

The material that follows explicitly addresses exposures to thorium. With respect to exposures 

to uranium, SC&A concurs with NIOSH’s conclusion that uranium exposures to workers during 

uranium operations and exposures to residual uranium subsequent to the termination of uranium 

operations at Dow Madison can be reconstructed in a scientifically valid and claimant-favorable 

manner. The basis for this conclusion is our understanding that the nature of the uranium 

operations that took place at Dow Madison during the covered period was similar to the uranium 

metal-working operations that took place at numerous other DOE/AWE facilities. We have 

reviewed numerous reports addressing these types of operations, including the reports by Harris 

and Kingsley (1959), Adley et al. (AEC 1952), and documentation in support of the Site Profile 

for Simonds Saw and Steel ORAUT-TKBS-0032 (ORAUT 2005) and the recently issued Site 

Profiles for Atomic Weapons Employers that Worked Uranium and Thorium Metals (BattelleTBD-6000 2006). We believe that these reports, taken in combination, are comprehensive in 

describing the range of operations and the range of exposures that workers might have 

experienced during uranium metal-working operations at DOE/AWE facilities during the early 

years. We believe that the material contained in these reports can be used to place plausible 

upper bounds on the uranium exposures experienced by workers at the Dow Madison facility. 

On August 6, 2007, while this report was being prepared, NIOSH issued Addendum to Dow 

Madison (SEC-00079) Special Exposure Cohort Evaluation Report (referred to here as the 

Addendum). The Addendum provides a review of approximately 700 pages of Dow documents 

that were placed on the “O” drive. SC&A has had an opportunity to briefly review the 

Addendum, and has thoroughly reviewed the Dow documents, and concurs with the factual 

material provided. SC&A also concurs with the Addendum conclusion that upper-bound 

external exposures to thorium at the Dow Madison facility can be reconstructed, based on a 

combination of a limited amount of external exposure data, and, more importantly, knowledge of 

the upper-bound exposures associated with working in close proximity and in contact with 

thorium metal and metal alloys, as determined using external exposure models, such as MCNP. 
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2.0 OPERATIONS AT DOW - MADISON 

During the period 1957 through 1960, and presumably before and after that period, Dow (and 

successor owners of the plant) produced magnesium alloys containing thorium for commercial 

customers.2 3

 Dow noted in its Annual Report for 1957 that, “Among promising new products 

are our new thorium-containing alloys which have greatly improved strength retention at 

elevated temperatures. They are being used principally in missiles and high-speed aircraft where 

they have distinct advantages over other metals in the 400 to 700 degree range” [emphasis 

added]. Some thorium sampling data from as early as September 1955 are available (Levy 

1959). 

Dow applied to the NRC for exemption from the licensing requirements under 10 CFR 20 (or 

10 CFR 40) for Mg-Th alloys requiring less than 4% Th. [A draft exemption letter was prepared 

by D. Cunningham of NRC on July 12, 1957, supporting this application (Cunningham 1957).] 

On July 14, 1958, AEC License No. C-2782 was amended to exempt Dow from the requirements 

of 20.203(e)(2) and 20.203(f)(2) of 10 CFR Part 20. 

Commercial use of Mg-Th alloys declined in later decades. ASM International observed the 

following in a 1998 publication (ASM 1998): 

Although thorium-containing alloys have found applications in missiles and 

spacecraft (Ref 14), they have lost favor because of environmental considerations 

and are generally considered obsolete. In Britain, for example, alloys containing 

as little as 2% of thorium are classified as radioactive materials that require 

special handling, thereby increasing the cost and complexity of manufacturing 

them. However, many thorium-containing magnesium parts are still in use, and 

some placement castings are still being produced; hence, some information on 

these alloys continues to be covered in various articles in this Handbook. 

A layout of the Dow Madison plant is included as Figure 1 (Cottrell and Williams 1990). Key 

locations include Building 7, where melting and casting were done; Building 6, where extrusion 

fabrication was done; and Building 5, where sheet and plate were rolled. A waste disposal area 

was located north and east of Building 7. Processing of magnesium-thorium (Mg-Th) alloys 

occurred in all of these major locations. Uranium work for the AEC was limited to Buildings 5 

and 6. Experimental uranium extrusion work was done in Building 6 and uranium rod 

straightening was done in Buildings 5 and 6. There are unconfirmed reports that some uranium 

rolling was done in Building 5. One document also mentions Buildings 152 and 356 (Silverstein 

1956b). Interviews with former employees could not identify these buildings and it is possible 

that they were at another Dow plant site. 

 

2

 According to Affidavit No. 9 (Dowpet 2006), work on Th-containing materials continued from 1951 through 

about 1998. 

3

 Commercial customers are those who purchase standard mill products based on normal terms and conditions. 

Commercial customers may be government entities. 
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Figure 1. The Dow Madison Plant 

Thus, to the extent that workers did not move from one production area to another during the 

1957–1960 timeframe due to job changes or nature of the job, workers in melting and casting 

(Building 7) would not have received significant uranium exposure. Workers in Buildings 5 and 

6, where uranium extrusion and, presumably, rod straightening were done, would clearly have 

received both uranium and thorium exposures during this period. 
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Interviews with former employees indicate that some worked their entire careers in the casting 

department, while some worked in several areas within the plant. 

2.1 Radioactive Materials Handled 

Alloys are identified by a code, which contains two letters and two numbers. The letters identify 

the metals in the alloy and the numbers the amount of metal, in percent, in the alloy. For 

magnesium-base alloys, H in the alloy descriptor indicates the use of thorium as an alloying 

agent, K indicates the use of zirconium as an alloying agent, and M indicates the use of 

manganese as an alloying agent. Alloy HK31 has a nominal composition of 3% Th and 1% 

(actually 0.7%) Zr. Worker Affidavit No. 2 specifically mentions alloys HK31 and HM21 

(Dowpet 2006; see also Affidavit No. 25). Other thorium-bearing alloys processed at the 

Madison Plant included HZ32A and ZH62A (Silverstein 1957a). The Dow Product Standards 

(July 1, 1968) list only three thorium-containing alloys; HK31A, HM21A, and HM31A (Dow 

1968). See Attachment A for additional details on Mg-Th alloys. Additional alloys were 

identified in the NIOSH Addendum. 

In addition, according to Affidavit No.1 (Dowpet 2006), uranium was processed on rolling mill 

#7 in the 1950s. While it is not totally clear from the affidavit, it appears that this was a one-time 

operation involving a single slab of uranium. No confirmation of uranium rolling has been 

uncovered to date in other documents. The affidavit also suggests that uranium rods were 

straightened either on the rolling mills or via extrusion. This testimony was from a second 

worker who started work in 1961. Further discussion of uranium rod straightening is provided in 

Dow 2007. The extrusion and straightening of uranium rods is elucidated in SRDB Ref ID 

3804.pdf, which is an excerpt from the closeout report for the FUSRAP Madison site. The report 

indicates that extrusion and straightening of uranium rods were conducted under contract to 

Mallinckrodt in the late 1950s and early 1960s. According to worker interviews, three methods 

of rod straightening were attempted in Buildings 5 and 6, but none were successful and the 

material was returned (Dow 2007). According to the FUSRAP report, “Records suggest that 

only a small quantity of uranium was involved in these operations.” The full text of SRDB 

Ref ID 3804.pdf, included here as Attachment B, describes Building 6 where the extrusion work 

was done. 

According to Worker Affidavit No. 2 (Dowpet 2006), the thorium was received in barrels and 

stored in the back of Building 7. This building contained the pot room, where various 

magnesium alloys, including Mg-Th alloys, were melted. Thorium was initially received as 

notched bars and later as pellets. According to Affidavit No. 9, the thorium bars came from 

England. (This is probably an error, since the AEC License specified import from Dominion 

Magnesium Limited in Canada). Silverstein (1957a) mentions that Dow also made a master 

alloy or “hardener” containing 25% thorium. However, it is not clear if Silverstein’s comment is 

related to Dow (Madison) or to Dow (Bay City). In its report of an inspection conducted by the 

AEC on August 4, 1960, the AEC inspector notes that thorium pellets, obtained from Canada, 

were stored within a fenced-off area in an isolated section of the Melt Room Building (USAEC 

1960). The fenced-off area had radiation warning signs and symbols. It is noted in the AEC 

report that 80 tons of thorium metal had been used through mid-1960 at the Dow magnesium 
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foundry.4 The inspector stated that “no items of noncompliance were observed or otherwise 

noted” during his visit to the plant in 1960. It is noted in the 1960 report that a prior inspection 

had been made from April 20–22, 1959. 

Details on the inventory of Th-bearing materials at various times, based on information 

contained in the annual AEC license applications, is included in Table 1. The 1957 license 

application includes an estimate of annual requirements for 135,000 lb of thorium metal and 

30,000 lb of ThF4 or ThO2. By the time of license renewal for 1960, the estimated requirements 

for ThF4 or ThO2 had dropped to 2,000 lbs, suggesting that the process of reducing thorium salts 

to make an Mg-Th hardener was being phased out. Also note the small quantities in inventory in 

1956, suggesting that commercial manufacture of Mg-Th alloys on a significant scale had not 

begun. 

Table 1. Thorium Inventory at Dow Madison5

Description Grade Th Content (%) Quantity (lb) 

October 31, 1956 Inventory 

Th metal – sintered pellets Pure 97 4.766 

ThF4 Commercial 71 22.702 

October 31, 1957 Inventory 

Th metal – sintered pellets Pure 97 6,254 

ThF4 Commercial 71 10,542 

October 31, 1958 Inventory 

Th metal – sintered pellets Pure 97 2,426 

ThF4 Commercial 71 9,700 

Th scrap Commercial 3 4,799 

Th scrap Pure 100 1,312 

October 31, 1959 Inventory 

Th metal – sintered pellets Pure 97 12,580 

ThF4 Commercial 71 9,480 

Th scrap Commercial 3 1,012 

Th scrap Pure 100 114 

2.2 Alloy Melting and Casting 

The Mg alloys were melted in cast iron pot furnaces. It is noted in the final affidavit in Dowpet 

2006 (the “Dow Employee Outreach Meeting” Affidavit, pg. 6) that some of the melting pots 

were 6,000 lbs, but it was later verified (Dow 2007) that this refers to the load in the pots. 

Apparently, the thorium was weighed on a scale and then introduced into the magnesium melt 

via a basket that permitted the thorium to be forced under the melt surface. According to 

 4

 It is not clear whether this is 80 tons of Th metal or 80 tons of Th-containing material. Elsewhere in USAEC 

1960, the following breakdown is provided: 64 tons as pellets from Canada, 13 tons in the form of scrap from 

Consolidated Edison Company, and 3 tons of master alloy containing 25%–40% Th as notched ingots from Canada. 

5

 It should be noted that estimates for annual thorium usage provided by worker testimony are much greater 

than estimates in this table (see discussions in Dow 2007). 
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Saunders (1960), the maximum frequency of weighing pellets during a casting run (using a 

scoop) is once per hour requiring 1 to 3 minutes. “The frequency of casting runs may be as high 

as once per month, extending to a maximum of five continuous days per run.” 

A “setting” for melting Mg-Th alloys consisted of a group of 10 melting pots. When alloying 

was completed in the required number of pots, the molten alloy was pumped to a continuous 

casting machine, where it was solidified and cut into slabs or billets. The slabs were then 

scalped prior to movement to one of the fabrication areas. Scalping involved removing about 

3/8 in of material from the slab surfaces, leaving a relative smooth, clean surface for subsequent 

fabrication. 

Based on the description in Affidavit No. 2, it appears that thorium was introduced directly into 

the melt. However, according to the generic description in Albert (1966, Chapter 3, Section 8, 

pg. 42), a master alloy containing equal parts of Th and Mg is first prepared, and this master 

alloy is used to obtain the correct composition in the Mg-Th alloy. Albert further notes that the 

maximum Th concentration in Mg-Th alloys is 4%. The use of master alloys is not mentioned in 

the Affidavits, but is mentioned in Dow internal documents (Silverstein 1956b, Silverstein 

1957a). It is possible that master alloys were used at Dow plants other than the Madison 

location. 

Hardener production at Dow involved the dissolution of ThF4 powder in a bath of molten KCl 

salt, and then reacting this mixture with molten magnesium (Silverstein 1957a). Originally, 

operations were conducted in Building 152 under a steel hood, where the airflow rate ranged 

from 50 to 100 linear feet per minute (lfm) and averaged 72 lfm. In some cases, two or three 

floor fans were added to increase the airflow in the vicinity of the reaction vessels to 80 to 

200 lfm, averaging 125 lfm at the hood (Silverstein 1956b). Air-sampling results are included in 

Table 2. Since workers from the Dow Madison plant could not identify Building 152, it is 

possible that hardener was produced at another Dow facility. As noted by Silverstein (1956b), 

Sample I-1 was a worst-case measurement: 

The actual hazard to workers would be less than indicated since workers 

normally would not be exposed continuously to concentrations shown, and would 

not occupy positions as close to the operations as the sampling locations. 

Concentrations of radioactivity have been added in brackets in Table 2 by the authors of the 

current review, based on the tolerance conversion factors presented in footnote a to Table 2. 
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Table 2. Air-Sampling Measurements during Mg-Th Hardener Production 

(Silverstein 1956b) 

Sample 

Number 

Date Description Sampling Time

& Volume 

Analysis a

I-1 4-11-56 Taken near two pots during 

the addition of ThF4 to molten 

KCl (1400°F). ThF4 dropped 

from hopper into stirred KCl. 

Position of sample was 

somewhat nearer the pot than 

the operator normally stands. 

10.0 min. 

(0.85 m3

) 

Thorium is 10 times tolerance 

[1.1 × 10-13 Ci/L], MsTh is 10 times 

tolerance [1.2 × 10-13 Ci/L], short-lived 

alpha activity is 3,000 times tolerance 

[3 × 10-8 Ci/L] 

I-2 4-11-56 Taken near workbench about 

30 ft. from pot. 20 min. after 

ThF4 addition completed. 

10.0 min. 

(0.85 m3

) 

Short-lived alpha activity is 50 times 

tolerance [5 × 10-10 Ci/L]. Th and MsTh 

are 1.5 times tolerance [1.8 × 10-14 Ci/L 

for MsTh and 1.6 × 10-14 Ci/L for Th]. 

I-3 4-12-56 Near workbench about in the 

center of the work area after 

ingots of Mg-Th hardener 

were poured; cleanup was in 

progress during sampling. 

10.0 min 

(0.85 m3

). 

Short-lived alpha activity 75 times 

tolerance [7.5 × 10-10 Ci/L]. Long-lived 

activity 70% of tolerance (Th and 

MsTh) [0.84 × 10-14 Ci/L for MsTh and 

0.77 × 10-14 Ci/L for Th] 

I-4 4-12-56 Near workbench 5 min. after 

ThF4 addition had begun. 

5.0 min. 

(0.425 m3) 

Short-lived alpha activity 8 times 

tolerance [8 × 10-11 Ci/L]. Long-lived 

activity is 40% of tolerance (Th and 

MsTh) [0.48 × 10-14 Ci/L for MsTh and 

0.44 × 10-14 Ci/L for Th]. 

a – Continuous exposure tolerances: mesothorium (MsTh) (Ra-228) – 1.2 × 10-14 Ci/L of air; short-lived alpha 

products (radon and thoron) – 10-11 Ci/L of air (Silverstein 1956b). Silverstein quotes the tolerance for Ra-228 as 

1 × 10-14 Ci/L of air in Silverstein 1957a. These are internal Dow standards. He also quoted the permissible level 

for thorium as 1.1 × 10-14 Ci/L (Silverstein 1957a). 

While not shown in Table 2, Silverstein (1957a) stated that use of local exhaust that drew air at 

150 lfm across the face of the pot reduced the long-lived alpha and beta emitters to 1/10 of the 

tolerance level (presumably this was 1.2 × 10-15 Ci/L for Ra-228 and 1.1 × 10-15 Ci/L for Th). 

Short-lived alpha radioactivity ranged from 10-14 to 10-12 Ci/L, values well below the tolerance 

limit of 10-11 Ci/L. 

The above discussion of hardener production is taken from documents, as referenced, from those 

available on the “O” drive for Dow (Madison). Further review of this material, and in light of 

the information gathered during the workers’ meeting in June 2007 (Dow 2007), SC&A is of the 

opinion that thorium hardener was not produced at Dow (Madison), but was probably used in the 

production of thorium alloys at Dow (Madison). Therefore, the data presented on hardener 

production may be irrelevant to this site, but it is included should further evidence come to light 

indicating differently. 

While it is questionable that thorium hardener was produced at Dow (Madison), according to 

worker testimony (Dow 2007), thorium hardener was used in the production of thorium alloys. 

However, because Silverstein 1956b describes a sampling location in Building 152, which is 

associated with Dow (Bay City), the data presented in this reference are likely from Bay City. 

Without evidence to the contrary, we believe these data are relevant to the melting process at 
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Health for factual accuracy or applicability within the requirements of 42 CFR 82. 

Dow Madison. Data on the use of hardener in melting of Mg-Th alloys are presented in Table 3 

(Silverstein 1956b). The samples described in Table 3 do not involve the production of Mg-Th 

hardener as summarized in Table 2; rather they involve the use of the hardener in producing 

Mg-Th alloys, such as HK31. Samples in Table 3 were taken on settings6

 in the SE corner of 

Building 152. (Since these samples were taken in Building 152, these measurements are believed 

to be from Bay City.) All of the samples in Table 3, except II-3 and II-6, were taken near two 

settings that had local exhaust ventilation away from the operator’s normal position with 90 lfm 

of airflow. Sample II-3 was representative of normal airflow in the melting room averaging 

30 lfm. Sample II-6 was taken over a setting with no local exhaust system, but the airflow was 

vertical at 40 lfm. (Concentrations of radioactivity have been added in brackets in Table 3 by the 

authors of the current review, based on the tolerance conversion factors presented in footnote a to 

Table 2.) 

Table 3. Levels of Radioactivity Associated with Melting Alloys Using Mg-Th 

Hardener 

(Silverstein 1956b) 

Sample 

Number Date Description Sampling Time and Volume Analysis 

II-1 5-4-56 Taken over a pot of molten Mg to which 

57.2 lb of Mg-24.5% Th hardener had 

been added. Hardener had been made 

from cell Mg and ThF4 3 weeks before. 

1,290°–1,400°F; normally held at 

1,400°F. 

5 min. 

(0.424 m3

) 

5 times tolerance for short-lived 

alpha material [5 × 10-11 Ci/L]. 

No thorium (Th) or mesothorium 

(MsTh). 

II-2 5-7-56 Taken over a pot of molten Mg at 

1400°F immediately after 53.4 lb of 

Mg-23.7% Th hardener had been added. 

Pot was stirred during sampling. 

Hardener was 3 weeks old. 

5 min. 

(0.424 m3

) 

4 times tolerance for short-lived 

alpha material [4 × 10-11 Ci/L]. 

No Th or MsTh. 

II-3 5-7-56 Taken near workbench 20 ft. from 

alloying pots. 143.4 lb of Mg-23.7% Th 

hardener, 3.5 weeks old, was alloyed at 

1,400°F. Melt was transferred to 

casting pot and then to a billet mold. 

6.5 hr. 

(33.0 m3

) 

5 times tolerance for short-lived 

alpha materials [5 × 10-11 Ci/L]; 

50% of tolerance for Th and 

MsTh. [ 0.6 × 10-14 Ci/L for 

MsTh and 0.55 × 10-14 Ci/L for 

Th]. 

II-4 4-10-56 Taken near pot in which 43.5lb of Mg25%Th hardener (3.5 weeks old) was 

alloyed with Mg at 1,350°F. 

4.0 min. 

(0.340 m3

) 

Short-lived alpha at tolerance 

[1 × 10-11 Ci/L], 1⁄2 tolerance for 

MsTh, no Th. [0.6 × 10-14 Ci/L 

for MsTh]. 

II-5 4-15-56 Taken near pot in which 29 lb of threeweek old Mg-25% Th hardener and 20 

lb of ten-month old Mg-27% Th 

hardener were alloyed with Mg at 

1,330°F. 

5.0 min. 

(0.425 m3

) 

Short-lived alpha 50% of 

tolerance [0.5 × 10-11 Ci/l]. 

MsTh 1.8 times tolerance 

[2.2 × 10-14 Ci/L]. No Th. 

 6

 A setting is a group of melting/alloying pots with interconnected piping that permitted the molten metal to be 

transferred from one vessel to another and to the continuous casting machine. 
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Table 3. Levels of Radioactivity Associated with Melting Alloys Using Mg-Th 

Hardener 

(Silverstein 1956b) 

Sample 

Number Date Description Sampling Time and Volume Analysis 

II-6 4-17-56 Over an open pot (1.5 ft above) in which 

16 lb of HK31 scrap (14 months old, 

3.14% Th ) was melted. 

9.0 min. 

(0.765 m3

) 

Short-lived alpha 18 times 

tolerance [1.8 × 10-10 Ci/L]. 

MsTh 65% of tolerance 

[0.8 × 10-14 Ci/L]. 

II-7 5-3-56 Near two pots in which 142.5 Lb of 

Mg-25% Th (3 weeks old) were alloyed 

with Mg at 1,400°F 

10.0 min. 

(0.85 m3

) 

Short-lived alpha three times 

tolerance [3 × 10-11 Ci/L]. 

No Th or MsTh. 

Additional details on the Mg-Th alloy melting process are included in the report of the AEC 

inspection in August 1960 (USAEC 1960). That information is excerpted as Attachment C to 

this report. 

The Affidavits do not provide much discussion about the use of fluxes or cover gas over the melt 

surface to minimize oxidation. When magnesium becomes molten, it tends to oxidize (burn) and 

explode, unless care is taken to protect the molten metal surface against oxidation. The 

following is noted on one website (http://www.key-to-metals.com/Article78.htm): 

Molten magnesium alloys behave differently from aluminum alloys, which tend to 

form a continuous, impervious oxide skin on the molten bath, limiting further 

oxidation. Magnesium alloys, on the other hand, form a loose, permeable oxide 

coating on the molten metal surface. This allows oxygen to pass through and 

support burning below the oxide at the surface. Protection of the molten alloy 

using either a flux or a protective gas cover to exclude oxygen is therefore 

necessary. 

Worker 20 in the “Dow Employee Outreach Meeting” Affidavit indicated that SO2 was used in 

the pot room presumably as a cover gas over the melt. 

Affidavit “Pot Room Conditions” states that exhaust fans existed in the pot room, but the fans 

often could not keep up with the smoky conditions in the area. (The worker who made this 

comment began work in casting in 1960, but it did not state whether exhaust fans were in use as 

of that date.) This problem was at its worst when alloying additions were made. Affiant No.1 in 

the “Dow Employee Outreach Meeting” Affidavit stated that there were eleven 4-ft ventilation 

fans in the pot room. Further discussion of pot room ventilation is contained in Dow 2007, and 

indicates that smoky conditions were often present in both the pot room and in the casting area. 

Testimony presented in this meeting describes the ventilation fans as being located at high points 

in the buildings, often inoperable, and ineffective. Albert (1966, Chapter 10, Section 7) states 

that canopy hoods are used over the Mg-Th melting pots, ensuring that there are no significant 

levels of airborne thorium or thoron.7

 This presumably generic description is at odds with that in 

 7

 This conclusion is based on a 1964 personnel communication between Albert and E.R. Averill of the 

Continental Insurance Company of New York, NY. 
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the AEC inspector’s report of August 1960 (see Attachment C, Section 15). Both of these 

descriptions are at odds with Silverstein (1957a), who stated, “Although no local exhaust 

ventilation is employed, the ceilings in the alloy area are 65 feet high and the natural ventilation 

is excellent.” 

Under Spectrulite ownership,8 several explosions occurred. It is not stated whether similar 

explosions might have occurred in the 1957–1960 time frame (Dowpet 2006, Affidavit – 

“Incidences involving explosions at the Spectrulite plant”). Worker interviews included in Dow 

2007 indicate that explosions were not uncommon. Silverstein (1957a) indicates that Mg fires 

were monitored during the period of interest for the present report.9 He provides information on 

three fires involving alloy HK31A. In the first fire, no thorium was found in the visible fumes or 

in the air a short distance away (breathing zone – 1 ft from fire). The air samples were collected 

and analyzed by three techniques to detect the presence of thorium; spectrographic analysis, 

x-ray diffraction, and x-ray fluorescence. Similarly, in the second fire, thorium was not detected 

in the fumes over the melt or at a distance of 5 ft from the fire. The absence of thorium was 

again based on chemical analyses (x-ray diffraction and x-ray fluorescence). Residue from the 

fire contained 10%–20% ThO2, as determined by x-ray diffraction. In the third fire, 

measurements taken from the fume indicated the presence of 2 × 10-9 μCi/ml of Ra-224 and 

2.5 × 10-9 μCi/ml of Pb-212. At a distance of 15 ft from the fire, measurements indicated 

8 × 10-12μCi/ml of Ra-224 and 2 × 10-11μCi/ml of Pb-212. Silverstein (1956b) summarizes 

results of additional air-sampling measurements made during Mg-Th alloy fires. Results are 

summarized in Table 4. (Concentrations of radioactivity have been added in brackets in Table 3 

by the authors of the current review, based on the tolerance conversion factors presented in 

footnote a to Table 2.) 

Table 4. Results of Air Sampling Measurements Made During a Mg-Th Alloy Fire 

Sample 

Number Date Description Sampling Time and Volume Analysis 

IV-1 5-8-56 Area sample 15 ft from a fire of 

4 lb of shavings (14 months old). 

No ventilation other than normal 

currents in Building 152. 

15.0 min 

(1.275 m3

) 

Short-lived alpha three times 

tolerance [3 × 10-11 Ci/L]. No Th or 

MsTh 

IV-2 5-8-56 In visible fume of the fire, 2 ft 

above the fire. 

1.0 min 

(0.85 m3

) 

Short-lived alpha activity 100 times 

tolerance [1 × 10-9 Ci/l]. MsTh 

3.5 times tolerance [4.7 × 10-14 Ci/L]. 

Worker 13 in the “Dow Employee Outreach Meeting” Affidavit (Dowpet 2006) stated that 

sludge, which collected in the bottom of the melting pots, was removed and placed in barrels and 

boxes. During the process, material would splash on the floor and fumes would rise up into the 

workers’ faces, making breathing difficult. This sludge was dumped on the property outside of 

Building 7. From time to time, workers were sent to pick through the sludge pile and recover 

 8

 As noted in Attachment B, Spectrulite purchased the facility in 1986. The magnesium assets were 

subsequently sold by the bankrupt company to Magnesium Elektron North America in 2003. 

9

 It is not clear whether these were accidental fires that started during production operations or fires 

intentionally started to characterize a disposal method for Mg-Th alloy scraps and chips (Dow 1958). 
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metal for remelting (Affidavit No. 2). Similar information was presented in the Dow 2007 

meeting. 

Silverstein (1956a, 1957a) described radioactive contamination measurements made during the 

production of 35,000 lb of alloy HM21. The area involved in making these measurements in 

Building 7 is depicted in Figure 2. The charge included cell magnesium and HM21 scrap from a 

previous run. After the magnesium and scrap had been melted, 190 lb of Th pellets and 150 lb of 

Mg-Th hardener were added to the melt. According to Silverstein (1956a), “Pots 32, 36, and 37 

were used for melting in scrap and alloying. The melt was then pumped into settling pot 33, to 

holding pot 31, to casting pot 30. It was then pumped into the DC [Direct Chill] unit mold.” 

Silverstein (1956a) further notes that normal ventilation conditions in the pot room were 

augmented by an open door and a circulating fan near Pot 36 (see Figure 2). Visible fumes rose 

vertically from the pots and returned to floor level 50 ft to 75 ft away. 
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Figure 2. Pot Room Schematic 
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The measurements made during the HM21 production run are summarized in Table 5 below. 

Table 5. Air Sampling Measurements Taken During Melting and Casting 

of Alloy HM21 at Dow Madison (July 22–23, 1956)

Sample 

Number Time Description 

Short-lived 

Radioactivity 

(Thoron & daughters)

(Ci/L) 

Radium-228 

(Ci/L) 

Thorium 

(mg/m3

) 

M-1 1701–1715 

7-22-56 

1.2 m3

Breathing zone (BZ) of man 

cutting open 10 drums of HM21 

heels with acetylene torch. 

HM21 was cast into drums on 

2-11-56. 

6 × 10-13 12 × 10-14

(10 x tolerance) 

0 

M-2 1800–1820 

1.7 m3

Between pots 36 & 37, 8 ft. 

away from either pot. HM21 

scrap (last melted on 2-11-56) 

was being melted, some burning 

of metal was observed. Workers 

occupied this area once every 5 

or 10 min for less than one 

minute each time. 

4 × 10-13 3.5 × 10-14

(3 x tolerance) 

0 

M-3 1842–1904 

1.87 m3

At workbench across aisle from 

pots 36 and 37. Workers spent 

most of their time here or outside 

door during melting down of 

HM21 scrap. 

5 × 10-13 0 0 

M-4 2027–2140 

6.2 m3

50 ft. south of operation near 

another group of settings on 

which men were working. 

5 × 10-13 0 0.004 

M-5 2214–2222 

0.68 m3

BZ of worker alloying 70# of 

thorium pellets into 5000# of 

HM21 + cell Mg in pot 32. 

Pellets burned before being 

immersed. 

6 × 10-13 0.27 × 10-14 0 

M-6 0036–0056 

7-23-56 

1.7 m3

In front of instrument panel. 

Pot 32 was recharged with 

HM21 scrap, melt was pumped 

from pot 37 to pot 32, and from 

33 to pot 31, during sampling 

period. Some burning occurred 

during pumping and also in pot 

32 when scrap was added. 

8.7 × 10-13 0.27 × 10-14 0.02 

M-7 0220–0250 

0.425 m3

Near washroom, about 100 ft. 

north of the group of settings in 

use. 

7 × 10-13 0.3 × 10-14 0 

M-8 0315–0320 

0.935 m3

BZ of worker alloying 50# of 

thorium pellets in pot 32. No 

burning of the pellets was 

observed. 

6.2 × 10-13 0 0 
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Table 5. Air Sampling Measurements Taken During Melting and Casting 

of Alloy HM21 at Dow Madison (July 22–23, 1956)

Sample 

Number Time Description 

Short-lived 

Radioactivity 

(Thoron & daughters)

(Ci/L) 

Radium-228 

(Ci/L) 

Thorium 

(mg/m3

) 

M-9 0624–0635 

1.87 m3

BZ of man alloying 100# of 

hardener into pots 31 & 32, 50# 

had been alloyed into pot 30 just 

before sampling began. 

Hardener was last melted on 

2-7-56. 

6.2 × 10-13 0.44 × 10-14 0.01 

M-10 0907–0929 

1.78 m3

BZ of worker attending mold 

during casting of HM21. 

8 × 10-13 0.17 × 10-14 0.011 

M-11 1007–1028 

1.78 m3

By billet caster 50 ft. south of 

operation. 70# of pellets were 

alloyed in pot 32, pot 32 was 

pumped into pot 31, pot 36 was 

mechanically sludged and pot 33 

was hand-sludged during 

sampling period. 

6.0 × 10-13 0 0 

M-12 1032–1053 

1.78 m3

By workbench across aisle from 

pot 37. Pot 33 was pumped over 

to pot 31, sludge was pumped 

from pot 32 into cake molds. 

7.5 × 10-13 0 0 

M-13 1107–1140 

2.7 m3

By instrument panel. Sludge and 

metal were pumped out of pots 

30 and 31 into cake molds. 

3.8 × 10-13 0 0.006 

M-14 1243–1310 

2.3 m3

By instrument panel. Sludge and 

metal were pumped out of pots 

30 and 31 into cake molds. 

3.8 × 10-13 0 0.006 

In a later survey made during the production of both HM21 and HK31, Silverstein (1957a) stated 

that no long-lived alpha or long-lived beta radiation was detected, and that short-lived alpha 

levels were in the range of 10-10 μCi/ml.10

A similar set of measurements was made during the melting and casting of HK31 on 

December 15-16, 1959 (Dow 1959). Details included in Attachment F indicate that 

concentrations of thoron and daughters were about an order of magnitude lower for the 1959 

measurements. The melting log from December 16, 1959, is summarized in Table 6. Pots 34, 

 10 Silverstein (1957) is not specific in his definitions of short- and long-lived alpha and long-lived beta. He 

states on page 6 that short-lived alpha samples in Table 1 displayed the characteristic half-life of radon daughter 

products. This suggests that Ra-224 was either not considered to be one of the short-lived alpha products or it was 

not released in measurable quantities. It can be inferred from Silverstein’s paper that Th-232 and Th-228 are the 

principle sources of long-lived alpha. It can also be inferred that Ra-228 is the primary source of long-lived beta. 

See Attachment E for details on the Th-232 decay chain. 
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35, 38, and 39 are not shown in Figure 2. It was noted in the melting log that the sludge recovery 

centrifuge operated on a 32-minute cycle throughout the day of December16. 

Table 6. Melting Log for Alloy HK31 

(December 16, 1959) 

Time Action 

9:00 Add 38 lb sheet Th to Pot 38 

10:00 Add 40lb Mg-Th master alloy to Pot 37

10:35 Add 65 lb of Th pellets to Pot 35 

11:00 Clean (sludge) Pot 37 

13:24 Pump Pots 32 and 33 

13:26 Charge Pot 32 

13:34 Charge Pot 37 

14:00 Charge Pot 36 

14:10 Add 95 lb Th pellets to Pot 38 

14:25 Add 50 lb Th scrap to Pot 37 

?? Add 9 lb Th to Pot 32 

Silverstein (1957a) also described some measurements of radioactivity in samples taken over a 

pot during the melting of alloy HK31 at 1,400°F as a function of time between initial melting of 

the alloy and remelting of the scrap from the initial melt. Regardless of the age of the alloy, 

significant levels of activity from short-lived alpha emitters were measured, while no long-lived 

alpha emitters were detected above the pot for any of the samples. This suggests that Th-232 and 

Th-228 with half-lives of 1.4 × 1010 yr and 1.91 yrs are not transferred out of the melt, based on 

the detection limits existing in 1957. (The Th-232 decay scheme is included as Attachment E.) 

Activity over the melt from short-lived alpha emitters was reported by Silverstein to range from 

0.5 to 18 × 10-7 Ci/m3. It is not known what short-lived alpha emitters were actually evolved 

during the melting process. The potential suite includes Ra-224, Rn-220, Po-216, Bi-212, and 

Po-212. It is likely that Rn-220, which is a gas, would evolve from the melt. Some fraction of 

the Po, which has a vapor pressure of 104

Pa at 1,346° F, might also be vaporized 

(http://en.wikipedia.org/wiki/Polonium). 

Mechanisms for removal of the other short-lived alpha emitters (Ra and Bi) are not obvious. As 

shown in Albert 1966, Figure 2.6, the Th-228 decay chain reaches equilibrium after about 

21 days, so that aging of the alloy for longer periods before remelting should not significantly 

alter the quantity of short-lived alpha emitters present. Samples of Mg-Th alloy aged 10 to 

14 months before remelting showed some increase in the quantity of radioactivity from longlived beta emissions. This may be the result of in-growth of Ra-228 from Th-232 decay (see 

Figure 3). Some of the Ra-228 could then escape from the melt and is captured in the sampler 

above the melting pot. However, the mechanism for such possible Ra emissions from the melt is 

not apparent. The information presented in this section indicates that Th-232 and Th-228 are not 

present in significant quantities in the air in the melting and casting area. Measurements 
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provided in Table 5 indicate the presence of 4 to 20 μg Th/m3 of air in those instances where 

long-lived alpha radiation is detected, but often no long-lived alpha is observed.11 

2.3 Alloy Fabrication 

Some of the Mg-Th alloys cast in Building 7 were fabricated into sheet via multiple passes on 

the rolling mills in Building 5. Apparently, up to seven mills were used with plate produced on 

mill #1 and sheets of various thickness produced on the other mills. At several stages in the size 

reduction process, the sheets were sanded to remove gouges or dirt and scale. Pickling was also 

used for surface cleaning (Dowpet 2006, Affidavit No. 2). Worker 15 in the “Dow Employee 

Outreach Meeting” Affidavit stated that on mill #1, there was no airborne dust. He opined that 

any generated dust would have been smothered by the mill coolant. 

Measurements of dust levels from various fabrication operations associated with the production 

of Mg-Th alloys at Dow are presented in Figure 3 based samples collected with an electrostatic 

precipitator and analyzed spectrographically for thorium (Albert 1966).12 Thorium 

concentrations as high as 53.4 μg/m3

 were measured in the dust from rotary filing operations. 

According to Silverstein (1957a), this high value was probably due to a large particle (called a 

brick) that was dislodged from the metal surface. He stated that such a particle would be of 

sufficient size as to not be breathed by the operator. In contrast, thorium concentrations of about 

1 μg/m3 were measured for rolling operations. Silverstein characterizes the samples used for 

Figure 3 as follows: 

All were breathing zone samples and the operation took place on equipment that 

is designed for safe operation with magnesium which means local exhaust and 

water traps to collect the magnesium fines, 

It should be noted in Figure 3 that the “Mill Slab Ovens” sample and the “Rolling Slabs” sample 

are based on area air samples, while the other samples in the figure are breathing-zone samples. 

For thorium that is freshly chemically separated, Th-232 and the Th-228 would probably be in 

radioactive equilibrium. Thus, for a dust sample containing 53.4 μg/m3

 of newly-separated 

thorium, the equivalent activities for Th-232 and Th-228 would be 5.9 pCi/m3

. As shown in 

Figure 4 (Albert 1966, Figure 2.5), during the initial time period after separation, the rate of ingrowth of Th-228 from Ra-228 is less than the rate of decay of Th-228, and the Th-228 activity 

declines relative to that of Th-232, reaching a minimum of about 40% after about 6 years. After 

that, the ratio of Th-228 to Th-232 activity begins to increase and the two radionuclides are again 

in equilibrium after about 55 years. 

11 Based on the fact that the mass ratio of Th-228/Th-232 is 1.34 × 10-10 at equilibrium and the specific activities 

for Th-232 and Th-228 are 1.1 × 105

 and 8.2 × 1014 pCi/g, respectively, the Table 5 measurements can be converted 

to a range from 0.44 to 2.2 pCi/m3

 for Th-232 and for Th-228. 

12 This same figure appears in Silverstein (!957) as Slide V, and in compliance report_0001.PDF as Exhibit B. 
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Figure 3. Results of Air Samples Taken During Processing Thorium-Magnesium 

Alloys 

(Albert 1966) 

It is expected that dust particles collected for the measurements presented in Figure 3 would be 

magnesium particles with the intermetallic phase Mg23Th6 precipitated within the grains and at 

the grain boundaries of the Mg crystals (see Attachment D for details). 

Results from three air samples taken on March 14, 1957, in the operator’s breathing zone during 

the hand-sanding of HK31 were as follows (Hoyle 1957): 

● < 3.5 μg/m3 Th 

● < 7 μg/m3 Th 

● 21 μg/m3 Th 

These values are well below the thorium “tolerance limit” of 100 μg/m3 used by Dow in 1957.13

(See also Attachment F, Section F.11.) 

 13 It appears that this level was reduced to 76 μg/m3

 by 1959 (Levy 1959). 
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Figure 4. Variation of Activity in Natural Thorium after a Single Purification 

Courtesy of Union Carbide Corporation, Oak Ridge, Tennessee (Albert 1966) 

Two additional air samples were collected on Millipore® filters on December 2, 1957, during the 

hand-sanding of HK31 with small air-operated vibratory sanders (Mitchell 1957). The sample 

showed significant alpha activity when counted on December 19, 1957. Assuming that all of the 

alpha activity was from Th-232, Silverstein (1957b) estimated that the thorium concentrations 

were 138 and 165 μg/m3

. However, Silverstein opined that the thorium activity was less than 

50% of the total, and said that spectrographic analyses were being conducted to verify the 

assumption. Although results of these spectrographic analyses were not included in the available 

documents, the Dow 1959 Annual Industrial Hygiene Survey (Levy 1959) quotes values of 18–

36 μg/m3

 for vibrator-sanding of HK31 during December 1957. It is highly likely that these are 

the thorium contributions to the total alpha activities measured on December 19, 1957. 

As described by Worker 21 in the “Dow Employee Outreach Meeting” Affidavit, the extrusion 

facility (Building 6) had six presses; two rated at 1,800 tons, two rated at 3,000 tons, one rated at 

5,500 tons, and one rated at 11,500/13,500 tons. The affidavits provide very few details about 

the extrusion process, e.g., alloys, shapes and sizes, surface clean-up, etc. Alloy HK was 

extruded on the “heavy press” after 1961 (Affidavit No. 8), but it is not clear whether extrusions 

of Mg-Th alloys were made prior to 1961. The final affidavit (Worker 21, pg. 11) indicates that 

extrusion work on some (unspecified) alloys began in 1953. A worker in the Dow 2007 meeting 
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indicated that thorium alloys were being tested in 1953, as well. Other workers have indicated 

that thorium alloys were produced as late as the late 1980s (Dow 2007). 

Pickling (chemical milling) was used to an unknown extent for surface treatment of mill products 

(Silverstein 1957a). The pickling tank was equipped with slot ventilation drawing about 50 lfm. 

Results from two samples taken during the processing of HK31 are presented in Table 7. 

Table 7. Air Sampling During the Chemical Milling of HK31 

Sample Analysis 

4 inches above tank in which HK31 was being acid 

pickled 

6.6 × 10-11 Ci/m3short-lived alpha; no long-lived 

alpha or beta. 

5 feet from pickle tank 4.5 × 10-11 Ci/m3 short-lived alpha; 2 × 10-12 Ci/m3

long-lived beta; no long-lived alpha. 

According to worker interviews (Dow 2007), pickling tanks were used in Building 6 and 

Building 7 at the Dow (Madison) plant. 

With regard to uranium fabrication, Worker 14 in the “Dow Employee Outreach Meeting” 

Affidavit stated that the work for Mallinckrodt was done on the Number 7 press. Other workers 

(Dow 2007) have indicated that uranium rod straightening took place in Buildings 5 (rolling 

mill) and Building 6. Mallinckrodt provided their own handlers and operators, and they would 

“pretty much clean up their mess after they were finished.” Apparently, Dow workers were 

present, but not active participants. Subsequently, as part of FUSRAP clean-up activities at the 

Madison plant, ORNL conducted a survey in March 1989 of Building 6, where extrusion work 

was done. This survey was in response to uranium extrusion and rod straightening work done 

for the AEC by Dow under a 1957 contract (extrusion)14 and a 1960 purchase order (rod 

straightening) from Mallinckrodt Chemical Company (Cottrell and Williams 1990). Eighteen 

dust samples were collected from overhead beams in Building 6 and analyzed for Ra-226, 

Th-232, and U-238. Ranges for the results are as follows: 

● Ra-226: 0.22 to1.3 pCi/g 

● Th-232: 0.48 to 7.8 pCi/g 

● U-238: 3.7 to 310 pCi/g 

In spite of the fact that only limited processing of uranium was done in Building 6, while Mg-Th 

alloys had been processed there for more than 30 years, the uranium contamination was far 

greater. This observation is consistent with other evidence presented in this report indicating that 

thorium mobility is low. Based on the sampled area (about 200 cm2), the dust measurements 

were converted to dpm/100 cm2 and compared with DOE guidelines. The Ra-226 contamination 

ranged from 40% to 46% of the DOE guideline, and averaged about 43%. The Th-232 

contamination ranged from 9% to 100% of the guideline, and averaged 45%. The U-238 

 14 The contract (No. 25034-M, March 15, 1957) provided for 12 monthly sessions of 28 hours each, including 

6-hr setup, 16 hrs of extrusion, and 6 hrs for clean-up (Cottrell and Williams 1990). It is not known whether all l2 

sessions took place. 
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contamination ranged from 10% to 1,360% of the guideline, and averaged 250%.15 The Cottrell 

and Williams report states that the Th-232 contamination found throughout the building is the 

result of separate licensed processes not connected with DOE (AEC) activities. 

Silverstein (1957c) observed that, based on a film badge survey, the average weekly external 

exposure to workers involved in a variety of production operations including foundry activities, 

inspection, and shipping was less than 10 mR with the maximum exposure being about 30 mR. 

It is possible that the author is referring to the results documented in Appendix F, Table F-5, 

since these are the only film badge data provided by Dow to NIOSH. 

External radiation exposure has been measured for a 2,000 lb16 stack of Mg-3.5% Th alloy 

(HK31A) slabs (40 in by 30 in by 7 in). The β/γ dose at the surface was 4.2 mR/hr, 1.8 mR/hr at 

1 ft, and declined to 0 at a distance of 7 ft from the alloy stack (Albert 1966, Figure 10.5; 

Silverstein 1957a, Slide 1). Dow estimated that a worker with hands in contact with and body 

1 ft from the alloy stack would receive an exposure to the hands of 168 mR/week and exposure 

to the body of 72 mR/week, based on a 40-hr week (Silverstein 1956c). These external 

exposures were less than the existing NCRP recommendation of 500 mR/week to the hands and 

100 mR/week to the body (Dow 1957). Dow noted that similar results were obtained on a stack 

of HK31 sheets, and observed that the beta component of the radiation was eliminated after 

crating (Silverstein 1957a). Details are provided in Attachment F, Section F.12 

Thorium pellets measured 35 mR/hr on contact and Dow estimated that, at the normal working 

distance from the pellets, the worker’s exposure was less than 1mR/hr (Sliverstein 1956a). In 

addition, Dow estimated that a single worker handling all the thorium pellets required for a 

100,000 lb run of Alloy HK31 would receive an external exposure of 53 mR (Dow 1957, Levy 

1957). Based on two to three such runs per year, the annual exposure would be 0.10–0.16 R 

(Levy 1957). 

Sludges remaining after melting Mg-Th alloys exhibited radiation levels of about 200 mR/hr at 

contact, but within a month, the radiation level had dropped to 10–20 mR/hr (Silverstein 1957a). 

HK21 scrap showed a maximum of 5 mR/hr at contact (Silverstein 1956a). Other measurements 

of external exposure are included in Attachment F (Sections F.8 [sludge dump], and F.9 [Th 

storage area]. 

The annual production of Mg-Th alloys is unknown, based on the available information. One 

worker involved in the manufacture of shipping crates estimated that every 2 months, Dow 

shipped four truckloads of metal to Rocky Flats, with each truck-load being about 36,000 lb 

(Affidavit No. 6).17 This is equivalent to 864,000 lb of alloy annually. (It is not known what 

Rocky Flats did with this large amount of material. Ulsh et al. 2006 sheds no light on this 

question. Given the information presented in Attachment G, shipments of such a large quantity 

15 The DOE guidelines for total residual averages were 100 dpm/100 cm2

 for Ra-226, 1,000 dpm/100cm2 for 

Th-232, and 5,000 dpm/100 cm2

 for U-238 (Cottrell and Williams 1990, Table 1). 

16 According to Silverstein 1957c, the stack weighed 3,000 lb. 

17 The worker who made this statement was employed at the Dow Madison plant from the early 1960s through 

the early 2000s. 
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of Mg-Th alloys to Rocky Flats appear questionable.) Assuming that the alloys shipped included 

equal quantities of HK31 and HM21, then the average thorium content would be 2.5%. Thus, 

these alloys would contain a total of 21,600 lb of Th. Other workers indicated that thorium 

alloys were shipped to Oak Ridge and Los Alamos, as well as contractors in the aerospace and 

defense industries (Dow 2007). 

An alternative guesstimate of Mg-Th production can be obtained from the August 1960 AEC 

Compliance Inspection Report, which indicates that as of July 1960, 80 tons of thorium had been 

used at the foundry (USAEC 1960). Assuming that this usage was over a 3 1/2-year period 

beginning in 1957, annual usage would have been about 23 tons. At an average Th content of 

2.5%, annual Mg-Th alloy would be about 920 tons (1,840,000 lb). This estimate seems high, 

since a 1957 Dow document (Levy 1957) estimates external exposures from two to three 

100,000 lb casting runs of Mg-Th alloys per year. Presumably, this represented production 

expectations at the time. 

Based on the information presented in Attachment A, the use of Mg-Th alloys has declined 

significantly since 1957–1960. By 1973, only about 5,000 kg of thorium oxide (9,700 lb of Th) 

was used in alloys for the entire aerospace industry. 

2.4 Sampling Techniques (circa 1956) 

2.4.1 Thorium Metal 

According to Silverstein (1956c), dust and fume was analyzed for thorium with a Mine Safety 

Appliance Company Electrostatic Sampler Model F using aluminum tubes. The thorium metal 

or oxide is recovered by dissolution in 75–80 ml of 10% nitric acid. The solution is evaporated 

to dryness and the residue is redissolved in nitric acid. A 0.03-ml aliquot of the sample is 

analyzed using the thorium line at 4019Å on a Baird 3-meter grating spectrograph with an Ennistype spark source. 

2.4.2 Alpha and Beta Activity 

Silverstein (1956c) describes the sampling process for alpha and beta activity as follows: 

...air samples were collected with an electrostatic precipitator and counted for 

alpha and beta activity in an NMC model PC-1 proportional counter. The 

counter efficiency for alpha was determined by counting a calibrated alpha 

source. Beta efficiency was assumed to be 50%, which is too low by an unknown 

backscattering factor. This error is on the safe side since reported beta counts 

are higher than actual counts. 

Silverstein (1957a) also noted that Millipore filter paper was used to a “small extent.” The 

samples taken in 1959, described in Table 5 above, also used Millipore filters. 

This description of the air-sampling techniques would indicate that measurements reported as 

short-lived alpha activity were not limited to thoron (which would not be readily collected using 
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these methods, because thoron is a noble gas), but would likely be comprised of the short-lived 

progeny of thoron, which are of a particulate nature and collected by the cited sampling methods.
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3.0 INTERNAL DOSIMETRIC IMPLICATIONS OF THORIUM 

OPERATIONS 

SC&A’s review of thorium operations and associated radiological data, as described in 

Section 2.0, reveal that we have a fairly complete understanding of the types of operations that 

took place between 1957 and 1960, and presumably this understanding is also applicable to 

post-1960 operations. However, until we have access to records for the post-1960 time period, 

this presumption is unverified. 

With respect to radiological data pertinent to reconstructing internal exposures, it is clear that the 

data are limited to a few air samples; however, these data are informative. The following 

sections present the ranges of airborne concentrations of Th-232 and its progeny observed during 

various aspects of the alloying operations, along with some discussion of the dosimetric 

implications of these measurements.18

3.1 Mg-Th Hardener Production 

The first set of data is provided in Table 2 and, as discussed in Section 2.2, these data apply to 

hardener production, which we believe took place at Bay City and not Dow Madison. 

Nevertheless, it is instructive to explore the implications of the measurements reported in 

Table 2. 

Four air samples of 5- to 10-min duration were collected in the immediate vicinity of the pots 

where hardener production took place, and also in the general work area. The thorium activity 

(which is assumed to be 50% Th-232 and 50% Th-228) was observed in one sample to be as high 

as 1.1 × 10-13 Ci/L, and in other samples, as low as about 1.1 × 10-15 Ci/L. To put these 

concentrations into perspective, it is useful to note that the 50-year committed effective dose 

equivalent (CEDE) for 1 micron AMAD, Absorption Type S aerosols of Th-232 and Th-228 are 

1.1 × 10-4 Sv/Bq and 4 × 10-5 Sv/Bq, respectively. Hence, the high end 50-year CEDE for 

Th-232 is derived as follows: 

D = 1.1 × 10-13 Ci/L × 1.1 × 10-4 Sv/Bq × 3.7 × 1010 Bq/Ci x 1.2 m3/hr × 

1000 L/m3

 × 1.0 × 105 mrem/Sv ÷ 2 = 27 mrem/hr of exposure 

For air samples collected in the general work area away from the pots, the airborne levels of 

thorium were observed to be about 1/100th of the observed concentration near the pots. At these 

locations, the dose rates would be about 100-fold lower. 

Because of the limited number of air samples and the fact they may not represent breathing zone 

samples, it is difficult to draw conclusions regarding these particular measurements. However, it 

would seem that the potential for internal exposures to Th-232 and Th-228 in localized areas in 

the immediate vicinity of the pots where hardener production took place could be high, at least 

18 Only the data presented in Section 2.0 are used in this analysis. More data are contained in Attachment F. 

While the ranges differ slightly from the data in Section 2.0, the conclusions drawn considering these data would be 

the same. 
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during the times when operations took place. Alternatively, it also appears that in the general 

working area (i.e. the work bench area), the potential for exposure to Th-232 and Th-228 was 

low. 

Table 2 also presents concentrations of Ra-228. The dosimetric implications of these 

measurements are as follows. The highest concentration of Ra-228 reported in Table 2 is 

1.2 × 10-13 Ci/L in the immediate vicinity of the pots. The 50-year CEDE for Ra-228 is 

1.7 × 10-6 Sv/Bq. Hence, the highest dose rate associated with the values reported in Table 2 is 

as follows: 

D = 1.2 × 10-13 Ci/L × 1.7 × 10-6 Sv/Bq × 3.7 × 1010 Bq/Ci × 1.2 m3

/hr × 

1000 L/m3 × 1.0 × 105 mrem/Sv = 0.9 mrem/hr 

In the general work area, the Ra-228 concentrations were somewhat lower. The implications are 

that exposures to Ra-228 were relatively low, as compared to Th-232 and Th-228. Table 2 also 

reports the presence of short-lived alpha emitters at relatively high concentrations. These 

radionuclides could include Ra-22419 (3.66 day half-life) and its progeny, which includes thoron 

(55.6 sec half-life) and its short-lived progeny. In the vicinity of the pots, the highest 

concentration of short-lived progeny of 3 × 10-8 Ci/L was observed. At the work bench area, the 

concentrations were observed to be about 60 to 375 times lower. Assuming that the short-lived 

progeny includes Ra-224 and all its progeny, a bounding dose from these isotopes could be 

estimated by assuming that 25% of the observed alpha activity was from Ra-224, and the 

remainder of the activity was from thoron and its progeny. This is considered bounding because 

Ra-224 has a much lower potential to become airborne than thoron during hardener production, 

and Ra-224 has a much higher dose conversion factor than thoron and its progeny, primarily 

because of its much longer half-life. On this basis, the following is an estimate of the dose rate 

(50-year CEDE) associated with the high-end concentrations of short-lived alpha emitters 

observed near the pots (assuming the presence of Ra-224): 

D = 3 × 10-8 Ci/L × 3.4 × 10-6 Sv/Bq × 3.7 × 1010 Bq/Ci × 1.2 m3/hr × 1000 L/m3

× 0.25 × 1 × 105

 mrem/Sv = 4.5 × 105 mrem/hr 

This is an extremely high dose rate. It is important to recognize that the short-lived alpha 

emitters may have been entirely thoron and its progeny, which have much lower inhalation dose 

conversion factors. For example the 50-year CEDE for 1 micron AMAD Pb-212 and Bi-212 (the 

major contributors to the dose associated with inhaling thoron and its progeny) are 1.9 × 10-8 and 

3.1 × 10-8 Sv/Bq. Therefore, if the short-lived progeny measurements reported in Table 2 

consisted primarily of thoron and its short-lived progeny, the dose rate, as calculated above, 

would be lower by about 100-fold. However, even this is an extremely high dose rate. 

Inspection of Table 2 reveals that the concentrations of short-lived alpha emitters were observed 

to be about 100 times lower in the work-bench area as compared to the measurements taken at 

19 Section 2 of this report provides information indicating that Ra-224 was not present in significant quantities 

in the short-lived progeny analyses. However, for the sake of completeness, this section includes an evaluation of 

the magnitude of the exposures if a significant portion of the short-lived alpha analyses included Ra-224. 
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the hardener production pots. Hence, exposure potential is further reduced by about a factor of 

100 to about 10 mrem/hr. This may still be a significant dose rate. 

Notwithstanding the limited amount of data and the considerable uncertainties, there appears to 

be a high potential for exposures associated with hardener production. However, it must be kept 

in mind that hardener production likely took place at the Bay City facility and not at Dow 

Madison. 

3.2 Melting of Alloys Using Mg-Th Hardener 

Section 2.2 also presents air-sampling data collected during the melting of alloys using Mg-Th 

hardener. While these data are believed to have been generated at Bay City, hardener was used 

at Dow Madison. A 6.5-hour air sample collected at this location observed Th-232 and Ra-228 

concentrations of 0.55 × 10-14 Ci/L and 2.2 × 10-14 Ci/L, respectively (see Table 3). These 

concentrations correspond to a 50-year CEDE rate of about 1 mrem/hr for Th-232 and about 

0.2 mrem/hr for Ra-228. 

For short-lived alpha, the concentrations were on the order of 1.8 × 10-10 Ci/L. Assuming this is 

predominately thoron and its short-lived progeny, the dose rate (50-year CEDE) was on the order 

of the following: 

D = 1.8 × 10-10 Ci/L × 3.1 × 10-8 Sv/Bq × 3.7 × 1010 Bq/Ci × 1 × 105

 mrem/Sv × 

1.2 m3/hr × 1000 L/m3 = 25 mrem/hr 

The implications are that the potential for exposure toTh-232 and Th-228 appears to be quite 

low. However, the potential exposure rate from thoron and its progeny was not insignificant. 

3.3 Mg-Th Alloy Fire 

As discussed in Section 2.2, explosions and fires were not uncommon at magnesium alloying 

facilities, but Th-232 and Th-228 were not detected in the fumes. However, as indicated in 

Table 4, short-lived alpha emitters were observed at concentrations as high as 1 × 10-9 Ci/L, and 

Ra-228 was observed at concentrations on the order of 4.7 × 10-14 Ci/L. These concentrations 

correspond to dose rates on the order of 140 mrem/hr CEDE and <1 mrem/hr, respectively. 

Also, Ra-224 and Pb-212 were measured in the fume at concentrations of about 2 × 10-9 μCi/ml 

(2 × 10-12 Ci/L). This corresponds to dose rates (50-year CEDE) on the order of 30 mrem/hr for 

Ra-224 and < 0.17mrem/hr for Pb-212. At a distance of 15 ft from the fire, the concentrations 

were observed to be lower by several orders of magnitude. The implications are that the 

potential for significant exposures were relatively small at the time of fire and explosions at MgTh alloying facilities, considering the magnitude of the observed concentration of thorium and its 

progeny and the relatively short duration of exposures during such incidents. 

3.4 Alloy Production 

Table 5 summarizes data characterizing the airborne concentrations of short-lived progeny, 

Ra-228, and Th-232/Th-228 during alloy melting. In general, the breathing-zone concentrations 
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of Th-232/228 were below the limits of detection, but some values as high as 0.02 mg/m3 were 

observed. This corresponds to about 0.08 Bq/m3 or about 2 × 10-15 Ci/L. This concentration 

corresponds to a dose rate (50-year CEDE) of <1 mrem/hr. Hence, it appears that the potential 

for exposure to Th-232 and Th-228 was extremely small during Mg-Th alloy melting and 

casting. 

The breathing-zone concentrations of thoron and its progeny were observed to be on the order of 

1 × 10-12 to 1 × 10-13 Ci/L. This corresponds to a dose rate of <1 mrem/hr. The breathing-zone 

concentration of Ra-228 was observed to be on the order of 5 × 10-15 Ci/L, but one sample was 

observed to be on the order of 1 × 10-13 Ci/L. The high-end concentration was observed for a 

worker cutting open a drum of alloy with an acetylene torch—an operation of 10 min duration 

(Silverstein 1957a). These values correspond to doses on the order of <<1 mrem/hr to about 

1 mrem/hr. Notwithstanding the limited data and associated uncertainties, it appears that the 

potential for exposure during alloying operations was small. 

Section 2 also discusses the results of the analysis of air samples collected over a pot during 

melting of alloy HK31 at 1400oF (see Table 4). The analyses were unable to detect the presence 

of long-lived alpha emitters (i.e., Th-232/Th-238); however, short-lived alpha emitters (thoron 

and its progeny) ranged in concentrations from 0.5 × 10-7 to 18 × 10-7 Ci/m3 (0.5 × 10-10 to 18 × 

10-10 Ci/L). These are high concentrations and are associated with doses on the order of tens to 

hundreds of mrem/hr CEDE if the exposures were sustained for such time periods. 

3.5 Alloy Fabrication 

Alloy fabrication consists of a broad range of mechanical operations, such as Mg-Th rolling. 

Thorium dust loadings ranging from 0.2 to 53.4 μg/m3 (6.9 × 10-17 Ci/L to 1.8 × 10-14 Ci/L) were 

reported by Albert (1966). Such concentrations are associated with dose rates ranging from 

<<1 mrem/hr to about 5 mrem/hr. As discussed in Section 2, the measured dust loading of 

53.4 μg/m3 is believed to be due to the presence of large, non-respirable particles, referred to as 

“bricks.” However, other measurements reported in Section 2 range from 18–36 μg/m3. As 

such, it appears that alloy fabrication operations have the potential to result in several mrem/hr 

(CEDE). 
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4.0 FOLLOWUP ON THORIUM EXPOSURES AFTER 1960 

At the Advisory Board Meeting held in Denver, Colorado, on May 2, 2007, SC&A was tasked to 

assess thorium exposures past the time period covered by the SEC petition (post-1960). In 

addressing this task, SC&A considered the material it had reviewed for accomplishing the 

reviews of the SEC petition and the NIOSH evaluation and documents made available on the 

“O” drive on May 1, 2007, from Dow Chemical Company. The relevant information from these 

sources is presented in Sections 1.0 and 2.0. 

With the understanding of the Advisory Board, SC&A did not attempt to obtain information 

from sources other than NIOSH or that distributed to both NIOSH and SC&A by the SEC 

petitioners and their representatives. As recognized by the Advisory Board, SC&A did not 

independently request or obtain information from any other source. Therefore, the content of 

this report is based on the knowledge obtained from the named sources and may not represent the 

full universe of information available from all relevant sources. With the exception of the 

materials from the workers’ meetings and affidavits (Dowpet 2006, Dow 2006a, Dow 2006b, 

Dow 2006c, and Dow 2007), the products on the “O” drive concentrate on the 1957 through 

1960 time frame. Therefore, most of the information for this discussion of thorium beyond 1960 

is extracted from the four workers’ meetings and affidavits filed with the petition. A subset of 

these affidavits related to thorium was provided to NIOSH by Simmons-Cooper on April 16, 

2007, and is available on the “O” drive. 

4.1 June 20, 2007, Workers Meeting 

The available documentary information focuses on the time frame of 1957 through 1960. The 

primary available source of post-1960 information on thorium alloy production and the attendant 

exposures is contained in the written records from the three Dow (Madison) workers’ meetings 

and the affidavits submitted with the SEC petition. Therefore, SC&A believed that we should 

attempt to more clearly define the particulars of thorium alloy production and exposure by 

meeting with Dow (Madison) workers to elicit information on their experiences in greater detail. 

Twenty-three former workers participated in the meeting and one offered testimony via the 

telephone. The workers represented a wide range of jobs in all departments of the plant, 

including, casting, extrusion, rolling mill, shipping, and laboratory testing. 

It should be recognized that workers’ testimony has both assets and liabilities relative to written 

documents when establishing a record of facts for events which occurred, in some instances, over 

50 years previous. On the asset side, there is some information that was never recorded or not 

now available that is only known to the workers. On the liability side, the recollections of 

workers for facts so far into the past can be flawed or altered by events or information gained 

since the time of the events. In addition, the workers were not privileged to some important 

information, particularly that pertaining to work involving radioactive materials, including 

thorium. That having been said, the workers knowledge of the facts relative to thorium alloy 

production at Dow (Madison) is a key element in understanding the exposure conditions that 

existed at the time of occurrence. 
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With the assistance of [Name redacted] and Simmons-Cooper, LLC, Chick Phillips of SC&A, 

accompanied by Grady Calhoun, NIOSH, met with former Dow (Madison) workers on June 20, 

2007, in Alton, Illinois. The meeting was held at the offices of Simmons-Cooper, LLC, the law 

firm supporting the petitioners’ SEC application. Simmons-Cooper provided the services of a 

court reporter, and the verbatim proceedings were made available to SC&A and NIOSH (Dow 

2007). Most of the information was provided by the workers, most of which was contained in 

the previous public meetings and the affidavits from the SEC application. However, some 

workers participating in the June 2007 meeting had not attended the public meetings, nor had 

they provided affidavits. 

This meeting was conducted with the following objectives: 

• Gaining a better understanding of the Dow Chemical operations relative to thorium and 

thorium alloy production, and the exposure conditions related to them 

• Identifying any thorium materials that may have been provided to DOE/AWE sites 

outside the 1957 through 1960 time frame 

• Identifying any radioactive materials, including thorium, that may have been processed 

at the Dow (Madison) site 

• Allow workers to present any information relevant to the SEC petition 

The format of the June 2007 workers’ meeting was intended to be topical, in order to address the 

objectives above. Each worker described their knowledge of thorium-related activities, 

including those related to the shipping of thorium to other DOE/AWE sites. They responded to 

questions from the SC&A representative, [Name redacted], Mr. Calhoun, and each other. The 

participating workers were encouraged to present any information that they believed to be 

relevant. The transcript from the meeting was reviewed upon its receipt on July 9, 2007. The 

meeting was very useful in clarifying previously available information related to the production 

and distribution of thorium alloys, which had been provided by workers in prior workers’ 

meeting and affidavits. However, no significant new facts regarding thorium alloy production, 

worker exposure conditions, processing of radioactive materials supplied by others, or 

distribution of thorium alloy products were offered. Points of emphasis for the workers included 

the following: 

• The absence of radiation monitoring or protective measures during the Dow era 

• The lack of knowledge imparted to the workers on the nature of thorium (or other 

radioactive) materials 

• The distribution of thorium alloys to other DOE/AWE sites (Mallinckrodt, Rocky Flats, 

Oak Ridge, and Los Alamos – with particular emphasis on Rocky Flats) 

• The provision of thorium alloys to aerospace and defense contractors 
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• The processing of presumably radioactive products from outside entities (DOE/AWE 

sites and aerospace/defense contractors) 

• The amount of airborne material present in casting and extrusion buildings during 

thorium alloy production 

• The large amount of overtime worked 

• The degree to which workers moved around the plant in different jobs 

4.2 Thorium Alloy Production and Distribution to DOE/AWE Sites 

Thorium alloys, namely HK31 and HM21, were produced by Dow Chemical Company and its 

successor companies at the Madison, Illinois, plant for a number of years. The length of time 

over which they were produced is not entirely clear. The site companies have had radioactive 

materials licenses allowing the possession of thorium from 1956 to the present. Documents of 

affidavits filed with the SEC application and from workers’ meetings indicate a varying estimate, 

which is understandable, since the terms of their employment vary (Affidavits 2006, Dow 2006a, 

Dow 2006b, Dow 2006c, Dow 2007): 

• “I was employed at the Dow Chemical plant from 1962 to 1996. ....... Thorium was run

periodically at the Madison plant throughout the years I worked there.” 

• “Thorium was run at the plant from 1957 to 1999.” 

• “I was employed at the Dow Chemical plant from 1960 to 1987 .... Thorium metal was 

being cast, extruded and rolled at the Madison Plant during my entire period of 

employment.” 

• “I was employed at the Dow Chemical plant from 1964 to 2002. ... I know that Thorium 

metal was being processed in (redacted) throughout the years I was employed at the 

Madison plant.” 

• “I was employed at the Dow Chemical plant from 1953 to 1995. Thorium ran 

periodically through the (redacted) at the Madison plant during this entire period.” 

' 

• “I worked at Dow Chemical from 1968 to 2002. We worked with thorium as long as I 

worked in the (redacted).” 

The best estimate, according to the testimonies of the Dow (Madison) workers, is that thorium 

alloys were being produced at the site from about 1956 (Affidavits, 2006) to the late 1990s, and 

possibly as late as 2002. 

Throughout the documents representing testimony presented at workers’ meetings and the 

affidavits associated with the SEC petition, workers indicate that they have knowledge that 
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thorium alloys HK31 and HM21 were supplied to other DOE sites, including Mallinckrodt, 

Rocky Flats, Oak Ridge, and Los Alamos, with a heavy emphasis on shipments to Rocky Flats. 

Excerpts from affidavits filed with the SEC application and from workers’ meetings include the 

following (Affidavits 2006, Dow 2006a, Dow 2006b, Dow 2006c, Dow 2007) (see Attachment 

G for additional information): 

• “I know that some of this thorium metal was shipped to Rocky Flats, Colorado.” 

• “One of my foreman told me that the thorium metal was to be shipped to Rocky Flats.” 

• “I was told that some of the thorium that we processed during this period was for the 

government and was being shipped to Rocky Flats.” 

• “I saw sales orders showing that the metal was being shipped to Rocky Flats.” 

• “This metal (thorium) was then packed to ship to Rocky Flats, Colorado. The scrap 

metal that the company there did not use was shipped back to our company to be melted 

down, and then the above procedure was started all over again.” 

• “We rolled and shipped thorium to Rocky Flats, Colorado, and other plants in the United 

States. We received thorium scrap material back from Rocky Flats, Colorado, and other 

plants.” 

• “And I also sent materials to Rocky Flats, Colorado.” 

• “We'd also send it (thorium alloy) to Los Alamos.” 

It should be noted that when the workers mention thorium metal, most often they mean thorium 

alloys HK31 and HM21. In an earlier workers’ meeting, there was some indication that thorium 

metal had been shipped to Rocky Flats. However, in the June 2007 meeting, there seemed to be 

a consensus that any thorium sent to Rocky Flats was in the form of the commercial alloys HK31 

and HM21. There is no firm evidence than thorium materials other than these alloys went to any 

DOE/AWE site, including Rocky Flats. 

Earlier in this report there is a discussion of the quantity of Mg-Th alloys received at Rocky Flats 

from Dow (Madison) (Section 2.3). Appendix G contains statements from workers at Rocky 

Flats relative to the use of Mg-Th alloys. While some workers at Dow (Madison) recollect that a 

vast amount of thorium-bearing alloys were provided to Rocky Flats, Rocky Flats workers who 

were in a position to know do not confirm such receipts and usage at Rocky Flats. 

4.3 Thorium or Other Radioactive Materials Processed at Dow Madison 

In addition to the uranium extrusions and rod straightening for Mallinckrodt, there are incidents 

as reported by the Dow (Madison) workers (Affidavits 2006, Dow 2006a, Dow 2006b, Dow 

2006c, Dow 2007) of special processing of radioactive materials at the Madison site. There are 
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several possible incidents of special (possibly radioactive?) materials receipt from Rocky Flats as 

reported by Dow (Madison) workers (Affidavits 2006, Dow 2006a, Dow 2007). 

The first incident is reported by a worker as taking place in 1955, when material was received 

accompanied by armed guards from Rocky Flats (Affidavits 2006, Dow 2006a, Dow 2007). The 

material was received to be processed in the rolling mill. The material was first believed to be 

plutonium and later platinum by the worker (Dow 2007). It is unlikely, because of security, 

criticality, and health and safety concerns, that plutonium would have been involved in such a 

shaping process. It is not known if Rocky Flats had a need for the rolling of materials such as 

platinum. The worker who related the incident indicated that a Geiger counter was used to 

facilitate cleanup following the rolling process. This would lead one to believe that the material 

was radioactive, and it is unlikely that Rocky Flats had radioactive platinum. 

A second event for special materials processing at Dow (Madison) reported by a worker is 

believed to have taken place in the 1955 time frame (Dow 2007). As in the first case, the 

material was accompanied by armed guards from Rocky Flats and was to be cut in a shear. The 

worker could not identify the material, except to recall it was too hard to cut in the shear. The 

worker remembered being told by the guard that the material was radioactive. The residue from 

the attempted shear was collected and returned with the material. 

At the August 22, 2006 meeting (Dow 2006c), a third worker stated that, “At some time, they ran 

plutonium through the plant, too, through the homogenating ovens.” The worker could not 

identify the time this occurred, and offered no further explanation of the origin or fate of this 

material. 

It is difficult when reviewing the entire set of records of workers’ meetings and the affidavits 

provided by them to discern the exact number of times the workers believe that special materials 

were run in the Dow (Madison) plant. It is not always clear if two workers are referring to one 

incident or more when they are relating the particulars of an incident. Therefore, there could be 

more than three incidents reported by Dow workers, but the above three seem to be the ones 

most consistently referred to by workers. 

There are several places in the workers’ meetings and the SEC affidavits where workers related 

the processing of special materials in support of aerospace and defense contractors. These are 

not considered relevant to the SEC petition process. 

4.4 Thorium Exposure Conditions beyond 1960 

In the documents provided to NIOSH on May 1, 2007, there were additional air-sampling data. 

These have been presented above in Section 2.2 and in Attachment F. No external monitoring 

data or bioassay data have been found for Dow (Madison) workers, and the workers have 

indicated that none were performed. Workers have stated that some film badge monitoring was 

done during the Spectrulite ownership, however, no attempt has been made to obtain these 

results. Just as is the case for the time period of 1957 through 1960, it would likely not be 

possible to reconstruct the doses resulting from radiothorium for Dow (Madison) employees 

beyond the 1960 time frame, for the same reasons. 
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SC&A can re-examine the Dow (Madison) worker exposure to thorium beyond the 1960 time 

frame should more information become available from additional sources. With no additional 

materials to illuminate the activities and exposure conditions involving thorium after 1960, there 

will continue to be a large gap in our knowledge.
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5.0 REFERENCES 

AEC (U.S. Atomic Energy Commission). 1952. Study of the Atmospheric Contamination in the 

Melt Plant Building. HW-23352(Rev.). Prepared by F.E. Adley, W.E. Gill, and R.H. Scott, Oak 

Ridge, Tennessee. 

Affidavits, 2006, Affidavits Obtained from Former Dow Madison Workers, submitted to OCAS 

as supplemental documentation supporting Petition SEC00079; November 27, 2006; SECIS 

REFID:10630 

Albert, R.E. 1966. Thorium – Its Industrial Hygiene Aspects. Academic Press, New 

York/London. 

ASM 1998. Magnesium and Magnesium Alloys. ASM Speciality Handbook. M.M. Avedieian 

and H. Baker, eds. ASM International/Materials Park, Ohio. 

Cottrell, W.D. and J.K. Williams 1990. Preliminary Results of the Radiological Survey at the 

Former Dow Chemical Company Site, Madison, Illinois. ORNL/TM-11522. Oak Ridge 

National Laboratory. SRDB Ref ID 3947.pdf. 

Cunningham, R. 1957. Exemption of Thorium Contained in 4% Thorium-Magnesium Alloys 

From Licensing Requirements Under Part 40 (Draft). U.S. Atomic Energy Commission, 

Attachment to Letter from R.F. Horlick to J.R. Delaney, subject: Materials Items Pending, 

July 12, 1957. pending item_00001.PDF. 

Dow 1958. Magnesium-Thorium Alloys: Industrial Health Experience in Fabrication and 

Production. Bulletin Number 141-170. The Dow Chemical Company, Magnesium Department. 

TDCC 0000167-000190. 

 

Dow 1959. Industrial Hygiene Sampling Report. December 15-16, 1959. TDCC 000212-000223. 

 

Dow 1968. Product Standards – General Product Information. A-1-1, Magnesium. The Dow 

Chemical Company. July 1, 1968. TDCC000443-000444. 

Dow 2006a. Transcript of Dow Workers Outreach Meeting, Pohlman Reporting Company. 

July 21, 2006. DOW072106NON. 

Dow 2006b. Transcript of Dow Workers Outreach Meeting, Pohlman Reporting Company. 

August 11, 2006. DOW081106NON. 

Dow 2006c. Transcript of Dow Workers Outreach Meeting, Pohlman Reporting Company. 

August 22, 2006. DOW082206NON. 

Dow 2007. Transcript of Dow Workers Outreach Meeting, Sandra J. Rynders, Court Reporter. 

June 20, 2007. 
Effective Date: 

August 24, 2007 

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0 - Draft 

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39 of 66 

NOTICE: This report has been reviewed for Privacy Act information and has been cleared for distribution. 

However, this report is pre-decisional and has not been reviewed by the Advisory Board on Radiation and Worker 

Health for factual accuracy or applicability within the requirements of 42 CFR 82. 

Dowpet 2006. Special Exposure Cohort Petition – Dow Chemical Company. Submitted to 

Office of Compensation Analysis and Support, National Institute for Occupational Safety and 

Health by Simmons-Cooper, LLC, on November 27, 2006. 

Harris, W.B., and I. Kingsley 1959. “The Industrial Hygiene of Uranium Fabrication.” 

Reprinted from the A.M.A. Archives of Industrial Health, May 1959, Vol. 19, pp. 540–565. 

Hoyle, H.R. 1957. Memo to C.A. Mitchell: Results of Air Analyses Made at Madison During 

the Hand Sampling of HK-31 Alloy on March 14, 1957. The Dow Chemical Company. July 8, 

1957. TDCC 000047-000050. 

Levy, D.J. 1957. Memo to G. Ansel, Technical Director: Current Exposure to Radioactivity. 

The Dow Chemical Company. March 8, 1957. TDCC000045-000046. 

Levy, D.J. 1959. Memo to J.R. Burns: Annual Industrial Hygiene Summary. The Dow 

Chemical Company. May 21, 1959. 

Mitchell, C.A. 1957. Memo to L.G. Silverstein: Dust Samples Collected During Hand Sanding 

of HK31 Alloy. The Dow Chem9ical Company. December 9, 1957. TDCC 000166. 

NIOSH 2007. SEC Petition Evaluation Report Petition SEC-00079, National Institute for 

Occupational Safety and Health. April 13, 2007.

ORAUT 2005. Site Profile for Simonds Saw and Steel, ORAUT-TKBS-0032, Rev. 00 PC-1, 

July 8, 2005. 

Saunders, W.P. 1960. Letter to Eugene D. McFall, Inspection Division, U.S. Atomic Energy 

Commission. The Dow Chemical Company. August 4, 1960. TDCC 000233 – 000235. 

Silverstein, L.G. 1956a. Determination of the Radioactivity Hazard Encountered in Alloying and 

Casting HM21 Alloy at the Madison Division. File:T2.1-6-1. Biochemical Research 

Department, The Dow Chemical Company. August 23, 1956. TDCC 000005-000013. 

Silverstein, L.G. 1956b. Health Hazards from Radioactive Decay Products of Thorium During 

Melting, Chemical Milling and Burning of Thorium Containing Magnesium Alloys. File: T34.3-

21-1. Biochemical Research Department, The Dow Chemical Company. June 19, 1956. TDCC 

000019-000030. 

Silverstein, L.G. 1956c. Magnesium-Thorium Alloys: Industrial Health Experience in 

Fabrication and Production. Biochemical Research Department, The Dow Chemical Company. 

December 1956. TDCC 000031-000042. 

Silverstein, L.G. 1957a. Industrial Hygiene Experience with Magnesium-Thorium Alloys. Dow 

Chemical Company. Presented at the Health Physics Society Meeting, Pittsburgh, 

Pennsylvania, June 19, 1957. USAEC Docket 40-17. 
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August 24, 2007 

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However, this report is pre-decisional and has not been reviewed by the Advisory Board on Radiation and Worker 

Health for factual accuracy or applicability within the requirements of 42 CFR 82. 

Silverstein, L.G. 1957b. Memo to C.A. Mitchell: Analysis of Air Samples for Thorium. The 

Dow Chemical Company. December 30, 1957. TDCC 000164-000165. 

Silverstein, L. 1957c. Is Radiation Safety A Consideration in Working Magnesium Thorium 

Alloys? Presented at the Magnesium Association Meeting, October 17-18, 1957, New York, 

New York. TDCC 000109-000148. 

TBD 6000, Site Profiles for Atomic Weapons Employers that Worked Uranium and Thorium 

Metals, Battelle-TBD-6000, Revision F0, Battelle Team Dose Reconstruction Project for 

NIOSH. December 13, 2006. 

Ulsh, B., B. Rich, and M. Chew 2006. Summary of NIOSH’s Investigation of Thorium Handling 

at Rocky Flats. December 21, 2006. (Thorium Report 12-27-06.) 

USAEC 1960. Compliance Inspection Report-Dow Chemical Company, Madison, Illinois, 

August 4, 1960. U.S. Atomic Energy Commission. October 11, 1960. Compliance 

report_00001.pdf. 

Young, Charles D. 1988. Record of Contact: The Former Dow Chemical Plant at Madison, 

Illinois. 11 May 1988. TDCC 000346-000347.
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Health for factual accuracy or applicability within the requirements of 42 CFR 82. 

ATTACHMENT A: MAGNESIUM-THORIUM ALLOY DESIGNATIONS 

(http://www.orau.org/ptp/collection/consumer%20products/magthor.htm) 

Magnesium-Thorium Alloy (ca. 1980s) 

Mag-Thor is a common name for magnesium 

alloys containing thorium. There are three 

typical compositions for such alloys: 

 magnesium-thorium-zirconium 

 magnesium-thorium-zinc-zirconium 

 magnesium-silver-thorium-rare earth metalzirconium

Notable properties of magnesium-thorium 

alloys include high strength, creep resistance 

at high temperatures, and light weight. 

For example, the alloy designated HK31A, 

typically formed into sheets or plates, 

contains 3% thorium and 0.7% zirconium. 

The “H” in the designation refers to the 

presence of thorium and the “K” refers to the 

presence of zirconium. It is described as 

having good castability, being pressure tight, 

and possessing creep resistance up to 350 

degrees centigrade (a reasonably high value). 

Mag-thor has had a variety of applications (e.g., in missiles, spacecraft, tanks), but its primary 

use is in the manufacture of aircraft parts, especially engines. According to NUREG-1717, the 

average concentration of thorium in these alloys was about 1.7%. 

There is a general trend away from the use of this material. Its radioactive content can be high 

enough that special procedures are required when handling it and disposal can be expensive. 

From 1973 to 1983, approximately 4,000 to 5,000 kg of thorium oxide was used per year in 

alloys for the aerospace industry. In 1991, only 500 kg per year was being used in all 

metallurgical applications, not just the aerospace industry, and by 1993 that number had dropped 

to 100 kg per year. At the time that NUREG-1717 was written (2001), there were only two U.S. 

manufacturers of magnesium thorium alloys (Wellman Dynamics Corp. and Hitchcock 

Industries) and they had indicated that they might be ceasing production. 
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Dose Estimates

Routine Use 

NUREG-1717 estimated a potential exposure of 50 mrem per year to an aircraft engine 

maintenance worker. The assumption was that the thorium content of the parts was 1.7%, that the 

thorium was ten years old, that there was a single large (75 kg) casting and ten smaller castings 

(1.6 kg each), that the worker was an average of 0.5 meters away from these parts, and that this 

exposure was for 40 hours per year with an additional exposure of 1000 hours per year to the ten 

small castings only. 

Distribution and Transport 

NUREG-1717 calculated a potential dose of 4 mrem per year to the driver of a truck transporting 

parts made of a magnesium-thorium alloy. The exposure rate at the drivers location was 

estimated to be 0.04 mrem per hour. 

The assumptions behind the calculations were that the parts contained 1.7% thorium, that each 

shipment consisted of 100 cartons with two 1.6 kg castings each, that the first row of cartons was 

2 meters away from the driver, and that the exposure was for 100 hours per year. 

Pertinent Regulations

10 CFR 40.13 Unimportant quantities of source material (2003) 

(c) Any person is exempt from the regulation in this part and from the requirements for a license 

set forth in section 62 of the Act to the extent that such person receives, possesses, uses, or 

transfers: . . . 

(4) Any finished product or part fabricated of, or containing tungsten or magnesium-thorium 

alloys, provided that the thorium content of the alloy does not exceed 4 percent by weight and 

that the exemption contained in this subparagraph shall not be deemed to authorize the chemical, 

physical or metallurgical treatment or processing of any such product or part; 

Last updated: 

Copyright 1999, Oak Ridge Associated Universities 
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ATTACHMENT B: EXCERPT FROM FUSRAP CLOSE-OUT REPORT 

FOR DOW MADISON SITE, SEPTEMBER 2001 

(SRDB Ref ID 3084.pdf) 

SITE DESCRIPTION AND HISTORY 

The Madison Site consists of a large, multi-sectional complex of 10 interconnecting buildings 

with a total under-roof area of about 130,000 square meters (m2) [1.4 million square feet (ft2)]. 

Building 6 is about 83-meters (m) [270-feet (ft)] wide and 303-m (1,000-ft) long. The main bay 

ceiling is approximately 14-m (46-ft) high, 18-m (60-ft) at the highest point along the building 

centerline. The building structure consists of steel columns on approximately 7.6-m (25-ft) 

centers, connected by trusses and multiple smaller vertical and horizontal cross members. Walls 

are concrete block with some brick veneer. Floors are concrete; with rough and pitted surfaces. 

Much of the floor in the vicinity of the extrusion press is covered with a thin layer of oily dirt 

and fine metal debris. 

The Madison Site located in Madison, Illinois, (see Figures 1 and 2) was used to perform

extrusions of uranium metal and straightening of extruded uranium rods for the AEC, the 

predecessor agency of the Department of Energy (DOE), during the late 1950s and early 1960s. 

The Dow Metal Products Division of Dow Chemical Company (Dow) conducted this work 

under subcontract to the Uranium Division of the Mallinckrodt Chemical Works (Mallinckrodt). 

The work was conducted in Building 6, a large multi-story metal building with a concrete floor. 

The adjoining Building 4 was used for material transfers. The AEC-funded research was 

conducted at the plant to determine what factors in the extrusion of uranium metal affected the 

selection of tools and auxiliary supplies to be used at a planned extrusion press to be located at 

another AEC production facility. The work included researching properties of various die 

metals, the contour of the die cavity, the nature of the lubricant to apply to the uranium metal, the 

composition of the “follower block” (the material placed between the uranium metal and the ram 

press), and the speed at which the metal could be extruded. At least two rod-straightening 

campaigns occurred at the Madison Site. Records suggest a small quantity of uranium was 

involved in these operations. Mallinckrodt retained accountability for the uranium throughout 

the operations and was responsible for removing unused uranium and for cleanup of facilities 

following operations. The AEC-funded operations resulted in residual radiological 

contamination in dust on overhead steel beams in the plant. 

In the Designation Summary for the Former Dow Chemical Company Site in Madison, Illinois 

(ORNL, 1990), the Department of Energy indicated that Dow also supplied materials (chemicals, 

induction equipment, and magnesium metal products) and services under purchase orders issued 

by Mallinckrodt. In March 1960, the Uranium Division of Mallinckrodt Chemical Works issued 

a purchase order for Dow to straighten Mallinckrodt-supplied uranium rods. Two rodstraightening campaigns were identified in the purchase order. One was to be completed on 

December 21, 1959, the second on January 25, 1960. Each campaign also included a cost for the 

cleanup of the area after each campaign. The actual periods of performance for this work and the 

actual quantity of uranium that was processed are unknown. However, the total value of the 

purchase order and the unit cost identified with the “lot size” indicates that the quantity of metal 

involved was most likely small. DOE indicated that no other operation or period of involvement 
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with the processing or handling of FUSRAP related material at the Madison Site has been 

discovered. 

Dow leased the Madison facility to Phelps Dodge Aluminum Corporation in 1969. Consolidated 

Aluminum Corporation assumed the lease in 1973 and exercised an option to buy the plant in 

1973. Consolidated Aluminum Corporation processed magnesium thorium alloys at the Madison 

Site. Consolidated Aluminum Corporation sold the Madison plant to Barnes Acquisition, Inc. 

[which appears to have been a subsidiary of the Spectrulite Consortium, Inc. (Spectrulite)] in 

September 1986. Although Spectrulite (the current operator of the facility) has also processed 

magnesium-thorium alloys at the site, these operations are beyond the scope of FUSRAP 

remedial actions and this document.
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ATTACHMENT C: SECTIONS 14, 15, AND 16 EXCERPTED FROM 

USAEC INSPECTION REPORT, AUGUST 4, 1660 

(compliance report_0001.PDF)

Note: The original document was not always completely legible, so the transcription presented 

here may have some minor errors. 

 

14. Program and Scope of Work 

The Dow Chemical Company is licensed to receive possession of and title to thorium metal 

and/or thorium compounds, both domestically and through import from Canada, for the use in 

the preparation of magnesium alloys at Madison, Illinois. 

Dow Chemical Company is further licensed to transfer and deliver possession of and title to 

refined source material to any person licensed by the Atomic Energy Commission, within “the 

limits of his license.” 

The largest supply of thorium comes from Canada in the form of pellets. As of July 1, 1960, a 

total of eighty tons of thorium had been used at the magnesium foundry at Madison, Illinois. A 

breakdown on the amount and type of thorium used at the foundry is as follows: 

(1) 64 tons in the form of pellets were obtained from Canada 

(2) 23 tons in the form of scrap were obtained from Consolidated Edison Company 

(3) 3 tons of master alloy ranging from 25 to 40 percent thorium in the form of notched 

ingots were obtained from Canada 

Thorium is stored inside a fenced off area in an isolated section of the Melt Room Building. The 

fenced off storage area and the Melt Room are posted with signs comprising the conventional 

radiation symbol in color and the words “Caution – Radioactive Material, Unauthorized Persons 

Stay Away.” 

In the melting area, 3% thorium is added to the magnesium melt. The magnesium-thorium alloy 

is poured in the adjacent Pour-Off Area. The slag and flux and are [sludged] off and are 

remelted. The metal is poured. Then the remaining sludge is poured off. It contains up to 50% 

thorium. The sludge is broken up, and the metal is recovered and remelted. 

The sludge containing from 4% to 8% thorium is taken to the Waste Area, where it is kept 

segregated for possible future recovery of the contained thorium. Fines resulting from grinding 

the alloy are [burned] at the Waste Area. The Waste Area is fenced off from the rest of the plant 

and is under constant guard. 

In the foundry, scrap from shipping, sawing, and milling magnesium-thorium alloy is kept 

segregated all through the plant for remelting. 
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At the time of the inspection there was approximately 7200 pounds of contained thorium and 

notched ingots of master alloy scrap stored inside the fenced off posted area inside the Melt 

Room Building. 

15. Facilities and Equipment 

The production area where two and three percent alloys are made in 20,000 or 30,000 pound lots, 

consist of a workbench, an instrument panel, a mold, and six large pots, all loaded in the Melt 

Room Building. (See Exhibit A.) 

Exhibit A 

Production Area 

Alloy 

Pot 

Settling 

Pot 

Scrap Melt 

Pot 

Alloy 

Pot 

Casting 

Pot 

Instrument 

Panel 

Holding Mold

Pot 

Door

Door

Work 

Bench 

Located sixty-five feet above the production area is a twenty-foot exhaust fan. Other 20-foot 

exhaust fans are located on each side of the production area. 

Adjacent to the Melt Building are other buildings and areas where a variety of mechanical 

operations are carried out. These operations are surface and open wheel grinding, rotary filing, 

buffing, sawing, drumming of powder and mill slab heat treatment and rolling. In each building 

and area where fumes are a problem, there are exhaust fans available to adequately remove the 

fumes. 

16. Radiation Measurements 

Radiation surveys along with a material inventory report are made each month and sent to the 

home office in Midland, Michigan. Radiation Evaluations are made of the waste storage area 
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where sludge containing up to 8% thorium is stored and the fenced off posted cage in the Melt 

Room Building where the virgin thorium is stored. Thorium is stored in the cage in such a 

manner as to prevent any large dose rate from emitting from the cage. The latest survey results 

which were made on August 4, 1960, are as follows: 

Waste Sludge Storage Area 

Location Open Window (beta) Closed Window (gamma) 

(1) 

(2) 

(3) 

(4) 

On secondary road 

50 feet from sludge 

At Radiation Sign about 

35 feet from sludge 

At fence – 80 feet 

north of sludge pile 

Surface reading of 

sludge piles 

.08 (mrem/hr)

.13 (mrem/hr)

.06 (mrem/hr)

10.0 (mrem/hr)

.06 (mr/hr)

.10 (mr/hr)

.04 (mr/hr)

8.0 (mr/hr)

Virgin Thorium Storage Area 

Location Open Window (beta) Closed Window (gamma) 

(1) 

(2) 

(3) 

At door of cage 

12 inches above 7000 

pounds of master alloy 

scrap 

Center of cage 

1.0 (mrem/hr)

7.0 (mrem/hr)

2.5 (mrem/hr)

.8 (mr/hr)

6.0 (mr/hr)

3.5 (mr/hr)

(Dose rate measurements were made with a Precision Radiation Instruments, Geiger Counter 

Model 121 with Model-B Beta-gamma probe.) 

Mr. L.G. Silverstein, the Radiological Safety Officer located at Dow Chemical Company’s 

Midland, Michigan offices, makes periodic visits to the Madison, Illinois plant to made dose rate 

evaluations of the two storage areas (waste thorium and virgin thorium areas) and to make 

toxicity and contamination evaluations of the breathing zone air inside the production buildings. 

He also makes recommendations to the Safety Department and to the Works Manager on 

radiological problems whenever they arise. 

Air samples taken recently by Mr. Silverstein were collected with an electrostatic precipitator 

and counted for alpha and beta activity in a NMC Model PC-1 proportional counter. The counter 

efficiency for alpha and beta were determined by counting a calibrated alpha and beta sources. 

Mr. Saunders stated that with the exception of welding, the common operations of fabricating 

magnesium do not contaminate workroom air sufficiently to be of concern. The safe practices 

normally followed to collect grinding dust, the standard storage methods employed for raw 

materials and finished products and the proper ventilation used in pickling operations all 

adequately control air contamination. 

Mr. Eugene Burnett, the plant’s safety advisor, stated that the Industrial Hygiene Standard for 

thorium is 76 micrograms per cubic meter of air. This standard is used when determining air 
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concentrations. (See Thorium Content, in Micrograms/Cubic Meter in Breathing Zone Air 

during several Mechanical Operations as Exhibit B.20) 

20 Exhibit B is not reproduced here but it contains the same information as included in Figure 3 of this report. 
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ATTACHMENT D: MG-TH SYSTEM 

(ASM 1999) 

The binary phase diagram for the portion of the Mg-Th system up to about 45 wt% Th is shown 

in Figure D-1. Over this composition range, a simple eutectic system exists with a eutectic 

temperature of 582°C and a eutectic composition of about 41 wt % Th. The eutectic consists of 

Mg with Th in solid solution and the intermetallic phase Mg23Th6. At the eutectic temperature, 

the maximum amount of Th in solid solution in Mg is 4.75 wt% (0.52 at %). Under equilibrium 

conditions, as the temperature decreases further, Mg23Th6 will precipitate from the solid solution. 

Figure D-1. Magnesium-rich portion of Mg-Th binary phase diagram 

(This is copyright-protected material) 

The diagram in Figure D-1 describes the situation when a metal solidifies under equilibrium 

conditions. In actual practice the situation may be different. As described in ASM 1999 (p. 28): 

Thorium. At the eutectic temperature of 589°C (1092°F), 4.5% Th is soluble in 

magnesium; however, because of alloy segregation, magnesium alloys containing 

as little as 2% Th often contain a divorced eutectic and show massive Mg4Th 

compound at the grain boundaries. At temperatures below the eutectic, this 

compound is also precipitated from solid solution. In castings, the precipitate 

forms within grains and is seldom visible. In worked structures, the precipitate is 

often clearly visible at grain boundaries. 
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The addition of thorium to magnesium-zinc alloys changes the degenerate 

eutectic, which contains magnesium-zinc compound, to a lamellar eutectic, which 

contains a magnesium-thorium-zinc compound. 

It is thus likely that any metallic particles generated during the processing of commercial Mg-Th 

alloys will be either Mg with a Mg23Th6 (or Mg4Th) precipitate within the particles or, for very 

fine particles, possibly only Mg23Th6.

ASM 1999. Magnesium and Magnesium Alloys – ASM Specialty Handbook. ASM 

International, Metals Park, Ohio. 
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ATTACHMENT E: TH-232 DECAY CHAIN 
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ATTACHMENT F: SUMMARY OF MONITORING RESULTS AT 

DOW MADISON 

This attachment summarizes the radiation monitoring results reported by Dow in various 

documents. Many of these results are also discussed in the main body of the report but are also 

included here for completeness. Dow frequently refers to radiation levels relative to “tolerance” 

in citing measurement results. “Tolerance” refers to an exposure level acceptable to Dow rather 

than a regulatory mandate. Tolerances used by Dow in 1957 (Silverstein 1957) were: 

● Thorium: 100 micrograms of Th per cubic meter of air21 or 1.1 × 10-11 microcuries per 

milliliter 

● Mesothorium (Ra-228): 10-11 microcuries per milliliter (1.2 × 10-11μCi/mL in Silverstein 

1956b) 

● Short-lived alpha emitters: 10-7 microcuries per milliliter (10-8 μCi/mL in Silverstein 

1956b) 

 

Monitoring results are presented in the following sections. 

F.1 

Date: July 22-23, 1956 

Activity: Melting of 35,000 lb of Alloy HM21 in Pot Room (see Section 2.2 above for 

additional details) 

Reference: Silverstein 1956, Silverstein 1957 

Results: Air Monitoring Data are included in Table F-1 

Table F-1. Air Sampling Measurements Taken During Melting and Casting 

of Alloy HM21 at Dow Madison (July 22-23, 1956) 

Sample 

Number Time Description 

Short-lived 

Radioactivity 

(Thoron & daughters) 

(Ci/L) 

Radium-228 

(Ci/L) 

Thorium 

(mg/m3

) 

M-1 1701–1715 

7-22-56 

1.2 m3

Breathing zone (BZ) of man 

cutting open 10 drums of HM21 

heels with acetylene torch. HM21 

was cast into drums on 2-11-56. 

6 × 10-13 12 × 10-14

(10 × tolerance) 

0 

M-2 1800–1820 

1.7 m3

Between pots 36 & 37, 8 ft. away 

from either pot. HM21 scrap (last 

melted on 2-11-56) was being 

melted, some burning of metal 

was observed. Workers occupied 

this area once every 5 or 10 min. 

for less than one minute each time.

4 × 10-13 3.5 × 10-14

(3 × tolerance) 

0 

 21 In a 1959 document the maximum allowable concentration was quoted as 76 μg/m3 (Levy 1959). 
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Table F-1. Air Sampling Measurements Taken During Melting and Casting 

of Alloy HM21 at Dow Madison (July 22-23, 1956) 

Sample 

Number Time Description 

Short-lived 

Radioactivity 

(Thoron & daughters) 

(Ci/L) 

Radium-228 

(Ci/L) 

Thorium 

(mg/m3

) 

M-3 1842–1904 

1.87 m3

At workbench across aisle from 

pots 36 & 37. Workers spent most 

of their time here or outside door 

during melting down of HM21 

scrap. 

5 × 10-13 0 0 

M-4 2027–2140 

6.2 m3

50 ft. south of operation near 

another group of settings on which 

men were working. 

5 × 10-13 0 0.004 

M-5 2214–2222 

0.68 m3

BZ of worker alloying 70# of 

thorium pellets into 5000# of 

HM21 + cell Mg in pot 32. Pellets 

burned before being immersed. 

6 × 10-13 0.27 × 10-14 0 

M-6 0036–0056 

7-23-56 

1.7 m3

In front of instrument panel. 

Pot 32 was recharged with HM21 

scrap, melt was pumped from 

pot 37 to pot 32, and from 33 to 

pot 31, during sampling period. 

Some burning occurred during 

pumping and also in pot 32 when 

scrap was added. 

8.7 × 10-13 0.27 × 10-14 0.02 

M-7 0220–0250 

0.425 m3

Near washroom, about 100 ft. 

north of the group of settings in 

use. 

7 × 10-13 0.3 × 10-14 0 

M-8 0315–0320 

0.935 m3

BZ of worker alloying 50# of 

thorium pellets in pot 32. No 

burning of the pellets was 

observed. 

6.2 × 10-13 0 0 

M-9 0624–0635 

1.87 m3

BZ of man alloying 100# of 

hardener into pots 31 & 32, 50# 

had been alloyed into pot 30 just 

before sampling began. Hardener 

was last melted on 2-7-56. 

6.2 × 10-13 0.44 × 10-14 0.01 

M-10 0907–0929 

1.78 m3

BZ of worker attending mold 

during casting of HM21. 

8 × 10-13 0.17 × 10-14 0.011 

M-11 1007–1028 

1.78 m3

By billet caster 50 ft. south of 

operation. 70# of pellets were 

alloyed in pot 32, pot 32 was 

pumped into pot 31, pot 36 was 

mechanically sludged and pot 33 

was hand-sludged during sampling 

period. 

6.0 × 10-13 0 0 

M-12 1032–1053 

1.78 m3

By workbench across aisle from 

pot 37. Pot 33 was pumped over 

to pot 31, sludge was pumped 

from pot 32 into cake molds. 

7.5 × 10-13 0 0 
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Table F-1. Air Sampling Measurements Taken During Melting and Casting 

of Alloy HM21 at Dow Madison (July 22-23, 1956) 

Sample 

Number Time Description 

Short-lived 

Radioactivity 

(Thoron & daughters) 

(Ci/L) 

Radium-228 

(Ci/L) 

Thorium 

(mg/m3

) 

M-13 1107–1140 

2.7 m3

By instrument panel. Sludge and 

metal were pumped out of pots 30 

and 31 into cake molds. 

3.8 × 10-13 0 0.006 

M-14 1243–1310 

2.3 m3

By instrument panel. Sludge and 

metal were pumped out of pots 30 

and 31 into cake molds. 

3.8 × 10-13 0 0.006 

F.2 

Date: Unknown but between July 23 1956 and June 19, 1957 

Activity: Production of Alloy HK31 and HM21 

Reference: Silverstein 1957 

Results: No-long-lived alpha or beta on any sample. Short-lived alpha in the range of 10-10 

μCi/mL.22

F.3 

Date: May 21, 1959 

Activity: Annual Industrial Hygiene Summary 

Reference: Levy 1959 

Results: A cumulative summary of all Madison survey to that date is included in Table F-2. 

Correspondence attached to Levy 1959 (Levy letter to H.R. Hoyle, July 27, 1959) suggested that 

it would be timely to evaluate thorium and daughter products evolved during sludge centrifuging 

since the first unit had been used in production for some time and second unit was being 

considered. According to worker interviews, the centrifuge(s) was only used for about two 

years. Because of an industrial accident, their use was discontinued (REFERENCE). 

 22 Silverstein (1957) is not specific in his definitions of short and long-lived alpha and long-lived beta. He 

states on p. 6 that short-lived alpha samples in Table F-1 displayed the characteristic half-life of radon daughter 

products. This suggests that Ra-224 was either not considered to be one of the short-lived alpha products or it was 

not released in measurable quantities. It can be inferred from Silverstein’s paper that Th-232 and Th-228 are the 

principle sources of long-lived alpha. It can also be inferred that Ra-228 is the primary source of long-lived beta. 

See Attachment E for details on the Th-232 decay chain. 
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Table F-2. Cumulative Summary: Madison Division Industrial Hygiene Surveys 

(May 21. 1959) 

Substance 

Maximum 

Allowable 

Concentration 

Concentration 

Found Location Date Comments 

Ra-228 1.2 × 10-14 Ci/l 0 - 12 × 10-14

Ci/L 

Pot room, alloying 

and casting HM21 

7/56 Present 

operation no 

problem 

0 – 3 Pot Room 9/55 

0.2 By #1 Slab Oven 10/55 Sl. Dust on 

HK31 Slab 

1 #7 Mill 10/55 Rolling HK31 

<.35 Sheet wire brush 

machine 

2/56 HK31 

<10 Slab Scalper 7/56 HM21 

Nil – 20 R.M. hand sanding 

HK31 

3/57 

18 – 36 R.M. Salvage 12/57 Vibrator 

sanders on 

HK31 

52 – 94 Tech. Dept. 3/58 Rotary sanding 

HK31 

Thorium 76 μg/m3 *

<9 - <15 #2 Mill 3/59 Rolling steam 

annealed HK31 

Thoron, etc. 

(short lived) 

10-11 Ci/l 3.8 – 8.7 × 

10-14 Ci/l 

Pot room, alloying 

and casting HM21 

7.6 Max. is only 

9% of 

tolerance 

* 1957 Dow documents used 100 μg/m3 as the Th tolerance limit. 

F.4 

Date: December 15-16, 1959 

Activity: Melting of Alloy HK31 

Reference: Dow 1959 

Results: Air Monitoring Data (from Millipore filter samples) included in Table F-3 

Table F-3. Air Sampling Measurements Taken During Melting and Casting 

of Alloy HK31 at Dow Madison (December 15-16, 1959)a 

Sample 

Number Time Description 

Short-lived 

Radioactivity 

(Thoron & daughters) 

(μCi/mL) 

Short-lived 

Radioactivity 

(Radon & 

daughters) 

(μCi/mL) 

Thorium 

(μg/m3

) 

1 1354–1458 

12-15-59 

1.37 m3

Work area in pot room, 6 to 8 ft 

from pots 37 & 38. Alloying 

HK31 

1.3 × 10-11 1 × 10-10 <26 

2 1351–1500 

1.54 m3

Work area next to control panel 

for sludge recovery unit which 

was not operating. 

3.1 × 10-11 0.73 × 10-10 <23 
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Table F-3. Air Sampling Measurements Taken During Melting and Casting 

of Alloy HK31 at Dow Madison (December 15-16, 1959)a 

Sample 

Number Time Description 

Short-lived 

Radioactivity 

(Thoron & daughters) 

(μCi/mL) 

Short-lived 

Radioactivity 

(Radon & 

daughters) 

(μCi/mL) 

Thorium 

(μg/m3

) 

3 1533-1553 

0.428 m3

Same as Sample 1. 2 × 10-9 2.3 × 10-10 <84 

4 1533–1554 

0.471 m3

Same as Sample 2. 1 × 10-10 1.2 × 10-10 <76 

5 0934-1034 

12-16-59 

1.28 m3

Pot room air near pot 37 & 38, 

about 7 to 8 ft distant. Rotameter 

dropped to 5 between 10:13 and 

10:26. 

1.2 × 10-11 1.7 × 10-10 <12 

6 0854-0954 

1.34 m3

Sludge recovery room next to 

panel on operator’s side. 

Rotameter on bottom at the end 

of run. . 

3.7 × 10-11 3.5 × 10-10 <11 

7 1038-1138 

1.28 m3

Same as #5 above. Ringstand 

knocked over. 

2 × 10-11 2.2 × 10-10 <12 

8 0958-1058 

1.34 m3

Same as #6. Rotameter at 55 at 

end of sample. Centrifuge 

started at 10:25 

0.5 × 10-11 3.1 × 10-10 <11 

9 1141-1241 

1.28 m3

Sampler next to telephone and 

instrument panel. Air flow down 

to zero at 12:40 

3.4 × 10-11 1.1 × 10-10 <11 

10 1101-1206 

1.46 m3

Sample on the centrifuge side ?? 

shield. Centrifuge started at 

11:28 with AZ31 sludge. 

3.2 × 10-11 2.6 × 10-10 <10 

11 1318–1413 

1.18 m3

Control panel near pots 30 and 

31.. 

1.6 × 10-11 1.9 × 10-10 <13 

12 1249–1349 

1.34 m3

Same as #10. Centrifuge started 

with HK31 at 1:26 

2 × 10-11 3.6 × 10-10 <13 

13 .Blank 

14 1355–1455 

1.34.3 m3

Sample on drum next to 

operator’s desk in sludge 

recovery. Batch of HK31 started 

at 2:25.. 

2.3 × 10-11 1.1 × 10-10 <11 

a – Long-lived beta activity above background was not detected in any samples except Sample 8. The value for this 

sample is illegible in the available copy but appears to be 2 x 10-10 μCi/ml or less. 

F.5 

Date: Unknown but prior to June 19, 1957 

Activity: Various mechanical operations 

Reference: Silverstein 1957 

Results: Air sampling results are summarized in Figure F.5-1. (See also Section 2.3 of main 

report). All samples except “Mill slab ovens” and “Rolling slabs” were breathing zone samples 
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taken during the rolling of HK31. The samples were taken with an electrostatic precipitator and 

spectrographically analyzed for Th. As described by Silverstein (1957): 

All [except as noted above] were breathing zone samples and the operation took 

place on equipment that is designed for safe operation with magnesium, which 

means local exhaust and water traps to collect the magnesium fines. The high 

sample in rotary filing probably resulted from a stray “brick;” a large particle 

which is typical of the operation, but would be breathed by an operator. 

 

Figure F.5-1. Thorium Dust Generated During Various Mechanical Operations 

on Th-Mg Alloys 
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F.6 

Date: March 24, 1958 

Activity: HK31 sanding 

Reference: Mitchell 1958 and Silverstein 1958 

Results: Air sampling data were also obtained during sanding of HK31 with a vertical 

metallurgical sanding disks on 2 in by 3 in samples. These samples were taken in the Technical 

Department metallurgical area (location unknown). Based on the sample size and location of the 

work, it is presumed that was small-scale experimental work rather than production work. The 

samples were collected on Millipore filters using a sampler operating at 0.5 cfm in the breathing 

zone of the operator. There was no local exhaust on the sanding equipment. Results are 

summarized in Table F-4. 

Table F-4. Thorium in Dust Samples Taken During Sanding HK31 

Sample No. Air Volume (ft3) Sanding Procedure Thorium 

Concentration (μg/m3

) 

1 2.7 4-grit sequence – 50, 

150, 320 and 0 

52.3 

2 1.5 150 grit only with 

periodic drag on wheel 

flange 

94 

3 1.5 320 grit only with 

periodic drag on wheel 

flange 

94 

F.723

Date: Unknown but prior to June 26, 1957 

Activity: Casting of HK31 

Reference: Silverstein 1957b 

Results: External exposures based on film badges worn by operators for 13 working days are 

included in Table F-5 

Table F-5. Film Badge Survey of HK31 Casting 

Production

Exposure (mr) Job min max

Pour Off 10 75 

Knockout <10 10 

Sandblast <10 10 

Bandsaw 10 50 

Chipper <10 10 

Machine Trim 10 10 

Inspection (rough) 10 10 

Pickler <10 <10 

Rotary File 10 50 

Buffer 10 10 

 23 After a detailed examination of Silverstein 1975b and other references, it is clear that these film badge results 

are for materials at Dow (Bay City). However, these results should be directly applicable to materials at Dow 

(Madison) because of their similarity. 
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Table F-5. Film Badge Survey of HK31 Casting 

Production

Exposure (mr) Job min max

Heat Treat 10 10 

Wheelabrator <10 <10 

Inspection (final) 10 10 

Touchup <10 <10 

X-ray <10 <10 

Fixture <10 <10 

Shipping 50 50 

Metal Reclamation 10 10 

Control Badge 10 10 

F.8 

Date: August 4, 1960 

Activity: Survey of Mg-Th sludge dump 

Reference: Saunders 1960 

Results: The posted sludge dump (east of a north-south road adjacent to Building 7) was 

surveyed with a Precision Radiation Instruments Geiger Counter, Model 121 (AEC #S6M93A) 

with Model B beta-gamma probe. The measurement data summarized in Table F-6 were 

submitted to the AEC. Based on these measurements, Dow concluded that the results indicated 

no problem and solicited AEC’s opinions relative to the significance of the data. 

Table F-6. Beta-Gamma Activity at Mg-Th Sludge Dump 

Exposure (mr/hr) 

Sample Location Cover Closed 

(excludes beta) Cover Open 

1 At hard road-secondary road intersection <0.05 --- 

2 On secondary road- nearest to location of sludge cakes 0.06 0.08 

3 At fence- south of dump 0.16 0.28 

4 Two feet from a pile 25’ x 10’ x 5’ 2 2.5 

5 Surface of above pile 7-8 10 

6 At fence – south of dump but 30 ft more easterly than 

Sample 3 

0.04 0.06 

7 At fence – extreme east of dump 0.04 0.04 

8 Surface of another pile 6-7 --- 

9 At location of “HM” sign 0.10 0.13 

10 Another pile 8 9 

11 Intersection of secondary road and path-road 0.05 0.06 

F.9 

Date: August 4, 1960 

Activity: Survey of virgin Th storage area 

Reference: Saunders 1960 

Results: External exposure measurements made at the posted cage where virgin Th was stored 

are summarized in Table F-7. At the time the measurements were made approximately 3,200 lb 

of contained Th was in storage. 
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Table F-7. External Exposure Survey of Thorium Storage Cage 

Exposure (mr/hr) 

Sample Location Cover Closed 

(excludes beta) 

Cover 

Open 

1 At door – near sign 0.8 0.9–1.0 

2 12 in above 7,000 lb of notched ingots of master alloy scrap 

(5 to 19.2% Th) 

5-6 6-7 

3 Center of cage area 2.5-3.5 --- 

4 Center of a cluster of eight 50-gallon steel drums each containing about 

350 lb of master alloy (27-42% Th) 

>20 

5 Immediate external surface of steel drum containing 170 lb of Th 

pellets 

>20 

F.10 

Date: September 17-18, 1956 

Activity: Melting and Casting of HM21 and HK31. 

Reference: Dow 1956 (TDCC 000014 - 000018) 

Results: Results of air sampling measurements made using electrostatic precipitator during the 

melting and casting of HM21and HK31 are summarized in Table G-8. Note: The copy from 

which this table was prepared was difficult to read and some of the interpretations and entries 

may not be accurate. ???? indicates illegible text. 
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Table F-8. Air Sampling Measurements Taken During Melting and Casting of 

HM21 and HK31 Alloys at Dow Madison (September 17-18, 1956)

Sample 

Number Time Description 

Short-lived Alpha 

Radioactivity 

(Ci/m3) 

Long-Lived 

Alpha 

Radioactivity 

(Ci/m3) 

Long-Lived 

Beta 

Radioactivity 

(Ci/m3

) 

101 1352–1452 

9-17-56 

5.1 m3

By control panel on west wall. 

Sludging Pot 38, melting scrap 

HK31 sheet in Pot 38, alloying 

hardener in Pot 35. 

1 × 10-10 none none 

102 1508–1538 

2.55 m3

Front of instrument panel. Scrap 

burned (????) and melted in Pot 32 

1 × 10-10 none none 

103 1545-1602 

1.275 m3

Sample no good 

104 1600–1624 

2.04 m3

By Pot 33. Mechanical sludging 

took place. 

2 × 10-10 none none 

105 1625-1647 

1.87 m3

Pot 35 melting scrap. ????? 2.3 × 10-10 none none 

106 1648-1652 

0.34 m3

Pouring ingots. 3.2 × 10-10 none none 

107 1733-1802 

2.96 m3

????? 1.2 × 10-10 none none 

108 ???? 

2.55 m3

????? 1.1 × 10-10 none none 

109 1933-1952 

1.7 m3

10 ft from Pots 35 and 36. Pumping 

from Pot 39 to Pot 26. Mechanical 

sludging of Pot 39 took place. 

1.74 × 10-10 none none 

110 0855-0928 

9-18-56 

1.46 m3

5 ft from Pot 35. Charged scrap 

sheet into Pot 38. Alloyed 150 lb of 

Mg-Th hardener into Pot 35. 

6.4 × 10-10 none none 

111 1005–1020 

1.275 m3

By workbench opposite Pot 35 

where ??? scrap was being melted. 

7 × 10-10 none none 

112 1015–1037 

1.87 m3

By workbench opposite Pot 37 4.7 × 10-10 none none 

113 ???? ???? ???? ???? ???? 

114 1249-1304 

1.275 m3

By ??? billet casting panel 2.2 × 10-10 none none 

115 1056-1121 

2.13 m3

4 ft from Pots 31 and 33. ???? 2.1 × 10-9 none none 

F11 

Date: March 14, 1957 

Activity: Hand Sanding HK31 Alloy (See also Table G-2 above) 

Reference: Hoyle 1957 
Effective Date: 

August 24, 2007 

Revision No. 

0 - Draft 

Document No. 

SCA-SEC-TASK5-0057 

Page No. 

62 of 66 

NOTICE: This report has been reviewed for Privacy Act information and has been cleared for distribution. 

However, this report is pre-decisional and has not been reviewed by the Advisory Board on Radiation and Worker 

Health for factual accuracy or applicability within the requirements of 42 CFR 82. 

Three samples were collected in the operator’s breathing zone during the hand sanding of a sheet 

of HK31 and analyzed spectrographically for Th. Sample No. 1, based on a 10-minute air 

sample at 1 ft3

/min, contained less than 1 μg Th per 0.283 m3. Sample No. 2, based on a 5-

minute air sample at 1 ft3

/min, contained less than 1 μg Th per 0.141 m3. Based on the 

measurements, these samples were judged to contain essentially no Th. Sample No. 3, based on 

a 10-minute air sample at 1 ft3

/min, contained 6 μg Th per 0.283 m3 or 21 μg/m3 of Th. 

Assuming a tolerance level of 100 μg/m3 for repeated 8-hour exposures, “It can therefore be said 

that there is no hazard to the health of workmen who are hand sanding sheets of HK-31 alloy 

under the conditions represented by these samples.” 

F12 

Date: January 10, 1956 

Activity: External Exposure to Mg-Th Alloys 

Reference: Dow 1956b 

External exposure measurements made with a Raychronix instrument on Mg-Th alloy sheets and 

slabs are summarized in Table F-9. The units of measurement are not stated in Dow 1956b but 

are inferred from other Dow documents to be mR/hr. 

Table F-9. External Exposure Measurements on Mg-Th Alloys 

Material Source 

Thickness (in) 

Distance from 

Source (in) 

Beta/Gamma 

Measurement (mR) 

Gamma 

Measurement (mR) 

0 2.8 0.5 0.25 6 1.2 0.6 

5.7 0 2.8 1.6 

9.3 0 2.9 1.7 

12-in squares 

HK31 sheet 

12 0 2.5 1.6 

0 4.7 2.4 

12 1.5 1.5 140 in x 53 in 

sheets of HK31 14 

48 1.5 

0 4.2 2.5 

12 2.0 1.8 

24 1.5 

36 0.5 

48 0.5 

40 in by 50 in 

sheets before 

crating 

7 

60 0.1 

0 2.5 2.5 

12 1.5 

24 1.2 – 1.3 

36 1.0 

48 0.5 

40 in by 50 in 

sheets after crating 

with cardboard 

and tarred paper 

7 

60 0.0 
Effective Date: 

August 24, 2007 

Revision No. 

0 - Draft 

Document No. 

SCA-SEC-TASK5-0057 

Page No. 

63 of 66 

NOTICE: This report has been reviewed for Privacy Act information and has been cleared for distribution. 

However, this report is pre-decisional and has not been reviewed by the Advisory Board on Radiation and Worker 

Health for factual accuracy or applicability within the requirements of 42 CFR 82. 

Table F-9. External Exposure Measurements on Mg-Th Alloys 

Material Source 

Thickness (in) 

Distance from 

Source (in) 

Beta/Gamma 

Measurement (mR) 

Gamma 

Measurement (mR) 

0 4.3, 3.5, 4.0. 3.2, 5.1, 

4.0. 3.5 2.2 

12 1.7 1.7 

24 1.2 1.2 

36 1.0 1.1 

48 0.5 0.7 

60 0.4 0.6 

72 0.2 0.2 

Slabs of HK31 10 

in by 26 in by 76 

in 

108 

84 0.0 0.0 

Attachment F References 

Dow 1956a. Industrial Hygiene Sampling Report. September17-18, 1956. TDCC 000014-

000018. 

Dow 1956b. Industrial Hygiene Sample Record – Madison. January 10, 1956. TDCC 000001 – 

TDC 000004. 

Dow 1959. Industrial Hygiene Sampling Record. December 15-16, 1959. TDCC 000212-

000223. 

Hoyle, H.R. 1957. Memo to C.A. Mitchell: Results of Air Analyses Made at Madison During 

The Hand Sanding of HK-31 Alloy on March 14, 1957. Biochemical Research Department , The 

Dow Chemical Company, July 8, 1957. TDCC 000047 - 000050. 

Levy, D.J. 1959. Memo to J.R. Burns: Annual Industrial Hygiene Summary. The Dow 

Chemical Company. May 21, 1959. 

Mitchell, C.A. 1958. Letter to L.G. Silverstein: Dust Samples Collected While Sanding HK31 

with Vertical Metallurgical Sanding Disks. The Dow Chemical Company, March 24, 1958. 

TDCC000193 

Saunders, W.P. 1960. Letter to Eugene D. McFall, Inspection Division, U.S. Atomic Energy 

Commission. The Dow Chemical Company. August 4, 1960. TDCC 000233 – 000235. 

 

Silverstein, Lawrence G. 1956. Determination of the Radioactivity Hazard Encountered in 

Alloying and Casting HM21 Alloy at the Madison Division. File: T2.1-6-1. Biochemical 

Research Department, The Dow Chemical Company. August 23, 1956. TDCC 000005-000013. 

Silverstein, Lawrence G. 1957. Industrial Hygiene Experience with Magnesium-Thorium Alloys. 

Dow Chemical Company. Presented at the Health Physics Society Meeting, Pittsburgh, 

Pennsylvania, June 19, 1957. USAEC Docket 40-17. 
Effective Date: 

August 24, 2007 

Revision No. 

0 - Draft 

Document No. 

SCA-SEC-TASK5-0057 

Page No. 

64 of 66 

NOTICE: This report has been reviewed for Privacy Act information and has been cleared for distribution. 

However, this report is pre-decisional and has not been reviewed by the Advisory Board on Radiation and Worker 

Health for factual accuracy or applicability within the requirements of 42 CFR 82. 

Silverstein, Lawrence G. 1957b. Letter to H.L. Price, Director, Division of Civilian Application, 

U.S. Atomic Energy Commission. June 28, 1957. TDCC 000362-000384. 

Silverstein, Lawrence G. 1958. Letter to C.A. Mitchell: Thorium Dust Samples Taken During 

HK31 Sanding. The Dow Chemical Company. April 16, 1958. TDCC000191. 
Effective Date: 

August 24, 2007 

Revision No. 

0 - Draft 

Document No. 

SCA-SEC-TASK5-0057 

Page No. 

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Health for factual accuracy or applicability within the requirements of 42 CFR 82. 

ATTACHMENT G: USE OF MG-TH ALLOYS AT ROCKY FLATS 

Source: NIOSH Evaluation of SC&A’s draft report on Other Radionuclides (thorium)

February 28, 2007. 

Telephone interviews were conducted January 9th and 10th with four former Rocky Flats 

personnel that started their careers at the facility in the 1950s, and each worked there for some 

25 to over 35 years. The four workers interviewed were in various management positions and 

would have knowledge of thorium uses and sources at Rocky Flats. The comments contained in 

this summary are specifically related to the use of magnesium alloy at Rocky Flats, and if these 

employees would have knowledge of any significant quantities (more than a few kilograms) of 

magnesium alloy present at the facility. 

One worker stated that the conveyor system in Building 776 box line used magnesium for 

reduction in weight and strength stability, and that no magnesium alloy was used in any 

processes for development or fabrication of weapons components. The worker had no recall of 

significant quantities or other uses of magnesium alloy at Rocky Flats. That worker also stated 

that if there were significant quantities of magnesium alloy present, [the worker] would have 

known about it. The worker also stated that pure magnesium salts were used in some of the 

chemical processes, primarily for reduction of metal. 

Another worker clearly remembered that only the pennants in the conveyor system were made of 

magnesium alloy. The conveyor was constructed of stainless steel. The worker stated that the 

conveyor system and pennants were fabricated and installed by an outside vendor. This worker 

recalled being one of the first persons back into Building 776 after the May 11, 1969, fire, and 

clearly remembered that only a few pennants that were no longer present. The worker estimates 

that each pennant weighed no more than 4 pounds, and there were about 10 to 15 pennants 

missing, primarily from Box B-4 in the North line; all others were present. There were about a 

hundred or so pennants in the box line. The entire box line was removed and sent as waste to 

Idaho. 

This worker also stated that there was no magnesium alloy used in any processes for 

development or fabrication of weapons components, and if there were significant quantities 

present, [this worker] would have known about it. 

A third worker did not remember exactly what the conveyor system was made of, but did 

remember that the pennants that held the parts on the conveyor were a special alloy. This worker 

recalled completing and filing a special record for the waste recovered after the 1969 fire to 

account for any fire hazard in the waste being sent to Idaho. The worker thinks it was 

magnesium. The most important point is that there was no magnesium alloy used in any 

processes for development or fabrication of weapons components. The worker also said the 

conveyor system and pennants would have been an outside vendor fabrication. The worker 

added that it would have been highly unlikely that Rocky Flats would have purchased the 

magnesium alloy from Dow Madison and provided it to the vendor as government furnished 

equipment (GFE). This worker also acknowledged the use of magnesium salts in metal 

reduction processes. 
Effective Date: 

August 24, 2007 

Revision No. 

0 - Draft 

Document No. 

SCA-SEC-TASK5-0057 

Page No. 

66 of 66 

NOTICE: This report has been reviewed for Privacy Act information and has been cleared for distribution. 

However, this report is pre-decisional and has not been reviewed by the Advisory Board on Radiation and Worker 

Health for factual accuracy or applicability within the requirements of 42 CFR 82. 

This worker also stated that if there were significant quantities of magnesium present at Rocky 

Flats, [this worker] would have known about it. The worker emphasized that because of the high 

degree of concern for fire potential, any materials used at Rocky Flats were reviewed and 

approved by the fire department prior to use. According to comments made during this 

interview, this worker would have known about any use of potentially flammable structural 

materials. 

This worker also added that the weapons component specifications at Rocky Flats were very 

precise and any materials used were carefully evaluated for possible effect on the neutronic 

properties of the weapons components. 

The fourth worker interviewed did recall that the conveyor line in B776 had some magnesium 

alloy, and some of it did burn in the May 11, 1969, fire. When asked for an estimate of the 

quantity of magnesium alloy in the box line, the worker said maybe a few hundred pounds at 

most, because “it goes a long way.” If there were significant quantities of magnesium alloy 

present, this worker would have known about it. This worker stated that there was no 

magnesium alloy used in any processes for development or fabrication of weapons components.